A laser power stabilization system for the μW probe laser is developed. Power stabilization is realized by automatically adjusting diffraction efficiency of the acousto-optic modulator. The relative intensity noise of the probe laser is effectively suppressed to the shot noise limit, and the long-term stability is better than 2×10-5 (1000 s). The loop function equation of the power stabilization system is deduced, and the suppression effect of the loop on the relative intensity noise is analyzed. The stabilized laser is applied to detecting the clock transition of the integrating sphere cold atom clock, and its contribution to the frequency stability is less than 1×10-13τ-1/2(τ is the samping time). The frequency stability of the clock is improved to better than 5×10-13τ-1/2.