Influence of incoming laser beam and environmental factor on mechanical effect of plasma shock wave was studied. The experiment was performed by focusing output pulses of the Nd:YAG laser at 1.06 μm wavelength with 10 ns pulse width and 320 mJ pulse energy on aluminum targets mounted on a ballistic pendulum. The impulse coupling coefficients Cm was determined as a function of target defocusing distance Z at 1.01×105 Pa. The impulse coupling coefficient increases slowly with the increasing of defocusing distance, and gets maximum quantity at defocusing distance about -12 mm. And then impulse coupling coefficient decreases sharply with the increasing of defocusing distance. The function at 4×103 Pa is similar to it at 1.01×105 Pa, except that the maximum at a larger defocusing distance. The intensity and the diameter of the laser spot are different at the different defocusing distance, so the rarefaction wave effect and the plasma shielding effect are different too. And the mechanical effects of plasma shock wave on target at different defocusing distance are also different. All these factors dominate the influence of defocusing distance on mechanical effect of plasma shock wave induced by laser.