This is a vast territory and many things can be said about it. However, in my mind the first thing migrants need to feel is that they are not lesser than the people who come to help them. See Albert Schweitzer quote: "The purpose of human life is to serve, and to show compassion and the will to help others."

The second thing is that if migrants come to a place they need to respect its ways as much as it needs to respect theirs.

The third thing is to develop means of communication, wherein learning the language is top priority. In this, children often learn language much faster than adults. So the question is how to build a system whereby children help their parents to learn, without losing their position in the family.

This is not an academic point of view.

As a young adult, I have been working with migrants. For me and I agree with Erez, the main point is to facilitate the learning of language skills for the children. But what is even more important is to teach the language and the culture to their parents. Most of the social skills are transmitted to the children by their parents.This is the key of a happy and successful life in the future. That doesn't mean that they will have to abandon their culture and their values.   

I will also give my personal opinion, not related with academic study.

Since migrants come to a new environment, they need to learn a lot of new. This is less problem for young children, because they can simply adapt to new environment in a school, that gives language learning along with subject learning.

The situation with adults is more difficult. They may have already education, and have no need to go to university once again. But many of them do not know language, and this is an important barrier. With age it becomes more difficult to study a new language (memory is not as good), and it often takes many years to learn it at a very intermediate level. And before this level becomes sufficiently high, it is difficult to teach migrants other important issues, like local culture and traditions .That is why clustering of many migrants (especially if they arrive in mass to a certain location) often takes place. They are becoming not integrated but segregated group and this can increase social tension.

It is less a problem for more qualified people who know one of international languages (like English) and can work on it. Jobs are fewer but they exist.

I know only one positive example with mass migration. It is return of Germans to Germany from the former USSR around 1990. In this case they could learn language and adapt to culture in almost homogeneous group, and it was quicker and easier. There was also not big problem with cultural adaptation, mostly how to live in free market economy.

thank you

migrant people( espicialy Afghan's) have mutual language and similar culture in Iran. here we have another problem and it,s about policy. so I'm looking for some theory to describe it.

I think that adaptation of migrants in similar cultural and language environment is much easier. Your example of Afghani people in Iran is similar to return of Germans. This can be successful integration, but the policy is clearly important. Here the problem is whether the state of Iran is willing to integrate so many migrants (or hopes that they are temporary).

P.S. I do not understand what you mean by theory here. There are many papers on migration theory. I also have a couple of them, but they seem to address a bit different questions - about optimal duration of temporal migration and about chain effect. But maybe you can find there some ideas useful for your work.

https://www.researchgate.net/publication/5112403_Migrants%27_Savings_Purchasing_Power_Parity_and_the_Optimal_Duration_of_Migration

https://www.researchgate.net/publication/222647408_The_dynamics_of_migration_in_the_presence_of_chains

Article Migrants' Savings, Purchasing Power Parity, and the Optimal ...

Article The Dynamics of Migration in the Presence of Chains