my mom used her newly applied B2 visa and went to the U.S. one week ago. She
has come to the U.S. many times, and everytime, she went back to China
within 6 months.
Today, I just find out: on her previous expired B2 visa, there is a stamp
saying "canceled without prejudice". What does it mean?
-"Cancelled without Prejuidice" means that the old VISA cancellation has been done as a matter of procedure, not because of anything wrong. In other words, they have cancelled the old visa as it is no longer needed because they have approved your mother a new B2 visa.
does it have any negative impact on my mom's future application for B2 and future entry to this country?
-Not at all.