I am married but pregnant with another man's child. Can I claim child support from this man. He knows it is his child. I am separated from my husband.

Most likely yes, So much will depend on where you live.

In general legally, your husband will be considered the legal father, so there will have to be paternity determined by DNA testing. Then the real bio father declared the legal father.

There may be a few areas where this will be harder and you may have to even go after your husband for the child support. You really need a local attoreny to review the local laws and help you with this.