This is not a good question. What are bilateral exchange rates? China and which other country are involved? Exchange rates of Chinese Yuan relative to the currency of the other country are the reciprocals of those of the currency of the other country relative to the Chinese Yuan. If the effect of one on the exports is negative the effect of the other will be positive, and vice versa. If the effect is insignificant, then there is nothing to worry about. Finally, there is nothing wrong if the effect of the exchange rates is negative and insignificant.

In gravity model, there are many explanatory variables such as GDP, Distance, Per Capita GDP, Exchange rates and some dummies. 

Toal sample size is China trade with 172 countries (1993-2016). If Exchange rates appeared with a negative sign and insignificant with Total trade, Export and Imports as a dependent variable.  

How can i Explain this Situation?

There is nothing wrong with the insignificant coefficient or a coefficient sign against the theory. Just you need an appropriate justification for it which can be found from the literature as well as by studying the trade patterns and flows between both countries.

Best Wishes