A seat adjusting mechanism of an exercise machine, of which a sliding rack is provided at bottom side of the seat that is moved relatively against the frame and an adjusting mechanism is arranged in-between a frame and a sliding rack of a seat, which is used to adjust and fix positions of the seat. The adjusting mechanism comprises a clamp-bearing member that extends to the frame and is installed on the location that the seat slides, and a clamping device that includes two clamp arms pivotally installed side by side on the sliding rack. The two clamp arms have their first ends being clamping parts that clamp on the clamp-bearing member, and their second ends being force-bearing parts that are receive foreign forces to have the clamping parts of the two clamp arms clamp with each other so as to clamp on the clamp-bearing member.