Abstract: A energy balance method of preamplifier system (PSA) is given based on the PSA in SG-Ⅲ facility. The preamplifier system has realized precise control of output energy by employing the method. The results of running of the facility show that the control precision of output energy of PSA is 5.47%. Furthermore, the influence factors of output energy fluctuation of PSA are quantitatively analyzed. The analytical results show that the primary influence factor of output energy fluctuation of PSA is the instability of 1 Hz pulse energy, and the key of enhancing the stability of 1 Hz pulse energy is to control the polarized energy fluctuation of the frontend seed.