A firing assembly for a surgical stapler includes a shell, a pressure chamber arranged in the shell to accommodates fluid, an ejector rod located below the shell to provide an actuated force and press the fluid in the pressure chamber, a number of piston holes communicating with the pressure chamber, a number of pistons received in the piston holes and a number of firing portions actuated by the pistons. The firing action of the stapler is gentle and the stapling effect for the anastomosed tissues is uniform.