The stability of output energy is an important parameter for ultra-short laser facility. To improve the energy stability of the front amplifier in SILEX-I ultra-short laser facility, the energy fluctuation of regenerative amplifier and pre-amplifier and their pump laser are measured by experiment. It reveals that the fluctuation of the pump energy has great influence on the stability of regenerative amplifier. It is the main cause of the energy fluctuation of the front amplifier. To solve this problem, the fluctuation of the main laser caused by the unstable pump laser is simulated based on Frantz-Nodvik equation and statistical method. Ways of improving the stability have been analyzed such as delaying the output, saturated amplification, and using multiple pump lasers. Stabilization is achieved by adopting an amplitude stabilizer. The root mean square (RMS) of energy output is lowered from about 4% to below 1%.