If a guy have affair with a married women an got pregnant can she file child support on him?

In the US it may make a difference as to where they live.

But in general yes

ANY question on law needs to include your general locale as laws vary by area.

If a woman has a child, she generally can and should apply for child support from the father. Paternity will need to be established by the court, but once it is, support will be awarded.

In general terms the answer is no. The reason being is there are laws in most states that make the husband the legal father. In being the legal father by law then they are the party responsible for child support. Most states have laws that work this way.

Now if there is a move to disestablish paternity by the husband or if it is not automatic then the OP would be responsible for child support so long as they are established the legal father.

The reason being is there are laws in most states that make the husband the legal father.

Good point, I hadn't considered that part of the issue. But the questions was "can" she file. The answer to that is yes, but first paternity would have to be legally established. If the woman and the husband do nothing then the bio father is off the hook.

Agree, but the issue is "can she" so in most states there are ways that it could happen, Husband dispute paternity in some, the bio father claim paternity in others, are all examples of possible ways she can.

As noted, request for more information as to where and what is happening is not presented.

In general terms the answer is no.

In 25 states answer is YES;)

Good point, I hadn't considered that part of the issue. But the questions was "can" she file. The answer to that is yes, but first paternity would have to be legally established. If the woman and the husband do nothing then the bio father is off the hook.

It's saying the same thing. Yes, she can file, the bio-father

could be defaulted, in which case CS would be ordered; or
could defend, saying she is married and thus her husband is presumed to be the father. In that case, she would have to prove (by paternity testing probably) that, no, the adulterous putative father is.