Recently, high beam quality, large energy pulsed lasers with variable reflectivity mirrors has increasingly applied in nonlinear optics, scientific research lidar, industry manufacture, and military etc. To obtain high beam quality and efficient extraction simultaneously from an unstable resonator, the super Gaussian mirrors peak reflectivity R0, the super Gaussian order n, and the round-trip magnification M are three key parameters which must be carefully selected. In this paper, intensive investigation on beam quality of unstable resonators with VRM affected by R0, n, M, R0Mn is presented by computer simulation. A optimum design resulting near diffraction limited beam quality, large energy, high repetition is obtained. Various VRM tested in Q-switched Nd:YAG lasers pumped by flashlamp. Less than 2 X LD, 6 ns, 40 Hz, 350 mj/pulse laser beams are obtained in single oscillator, more than 1 J/pulse is realized through one amplifier. The experimental results are in good agreement with the optimum design.