Abstract: Based on the theory of probability and statistics, the energy instability which caused by the random error from optical parameters of multi-beam high power solid laser was calculated in this paper. The simulation prototype has been founded on the dozens of four-pass amplifiers. The energy spread was evaluated for the amplifiers which output 1ns and 13ns flat-in-time laser pulses. As a result, the energy spread of outputing pulses for 1ns pulses was bigger than that for 13ns pulses at the same conditions. Because for 13ns pulses, the amplifiers have been forced to work at deep gain saturation state; but for 1ns pulses, the amplifiers have been worked at linear amplification state. Using this characteristic of amplifiers, the energy spread of multi-beam lasers could be reduced by increasing