I thought if have rainfall mean never take the soil samples from the fields

Yes, you can take. the soil samples

Yes you can collect the soil samples.

I have never token indisturbed soil samples with rainfall. I usually wait one or two days, to let the soil drainage. Here in SE Spain, most days are dry (no rain) so we don't need to take the samples the very same day we have a rainfall episode.. . Maybe with so low rainfall (below 3.5 mm) it is not so important.

I just sampled soils from cane plantation in Guangxi province in China under rain

Thank you very much for your valuable answer friends

Which kind of samples? Intact soil, aggregates or just bulk soil?

Is it correct to assume that there has not been rain for sometime (so the soil is relatively dry)...and at the time of sampling, it is raining around ~3.5 mm? If yes, then two things should be considered. If the soil is too dry, you may have incorrect characterisation of the structure (depending on which method u use). If you can sample in the rain without smearing the samples prior to analyses, then its okay. But it really depends on the field situation. You should assess if the current conditions makes it possible to take your samples with minimal disturbance.