Nanosecond (ns) pulsed laser with high average power and high repetition rate is a potential solution for laser cutting, laser welding and many other processing applications. With the increase of pulse repetition rate, especially when it is higher than 50 kHz, it is difficult to accumulate enough upper state population in a limited time, and the stability of laser pulse becomes a challenge in the design of laser. The main oscillator power amplifier (MOPA) system is the main method currently used. It is difficult to obtain ns pulsed laser with high average power, high repetition rate and high beam quality through direct oscillation. In this paper, the quantitative relationship between the intensity stability of laser pulse and the pump rate is analyzed according to the simulation of high repetition rate Q-switched laser. Combined with the use of plane-concave lenses to make the cavity a thermal-near-unstable cavity with a large-volume fundamental mode, the balanced design of Nd:YAG acousto-optic (AO) Q-switched laser oscillator with high repetition rate, high power and high beam quality was realized. A direct oscillation of 100 kHz high power high beam quality ns pulsed laser by using side pumped module was realized for the first time, the discrete coefficient of pulse intensity was only 0.041, the output power exceeded 142 W, the pulse width was 165 ns, and the beam quality factor M² was 1.5.