Question
I have a field need to use earth worm, that's why I can't use any chemical liquid/item to kill my grass from the field. Do you have any idea what kind of product can get rid / kill my grass but does not harm to my earth warm. Thank you

Answer
You have posed a very interesting challenge.
You do not say how large the area is and I gather that you are using it as a earthworm farm. Based on these two factors, your options appear to be:-
Rent or lease the area for grazing rights. This presumes that the present crop cover is suitable for animal grazing and that there are interested farmers/ranchers nearby.
Grow a (legume) crop that is suitable for your location. If this crop can be sold, so much the better. If not, it can be incorporated into the ground for added amelioration of the earthworm habitat.
Weed removal would be limited to:
Manual methods e.g. hoes, machetes and fork.
Light mechanical methods might include string trimmer and shallow rotavators.
Remember that Mother Nature will 'heal' bare ground with some sort of greenery.
I do not know how earthworms on a commercial scale might coexist with birds so I would caution against any activity which will attract avian interest.
I am sorry that I cannot be any more specific but I shall be very interested to know how you resolve the problem. It is certain to elicit wider interest.