A circular stapler with an automatic safety mechanism comprises a stapler body, a stapler head, an actuating mechanism and a transmission adjusting mechanism; wherein the actuating mechanism comprises a handle and a handle coat, the transmission adjusting mechanism comprises a screw rod and an adjusting knob; a stopper is disposed either inside said handle or said handle coat; a safety rod and a pulling sheet are further provided, the safety rod has a safety block on one end and has a protruding block and a pin hole as its middle portion, a cylindrical pin is inserted into the pin hole so as to get the safety rod to be pivotally connected with the stapler body; the pulling sheet has a rear portion mounted on the screw rod and has a pulling hook at its front portion, the pulling hook hooks with the protruding block.