An impact device for power transmission includes an impact unit and a hydraulic pressure adjusting unit. The impact unit has a hydraulic cylinder which is installed with a piston assembly and an output shaft therein for outputting torque through the output shaft by utilizing a hydraulic pressure provided in the hydraulic cylinder. The hydraulic pressure adjusting unit is disposed in the hydraulic cylinder and operative to adjust the hydraulic pressure of the hydraulic cylinder. Therefore, the impact device for power transmission can generate a hydraulic torque pulse, and the magnitude of the output torque can be adjusted by the hydraulic pressure adjusting unit.