In this paper, we optimize the amplification efficiency of a nanosecond pulse CO2 laser in a fast flow amplifier using dual-band and multispectral lines techniques. Utilizing a six-temperature model and a random rotational relaxation model, we simulate the time-domain amplification process of dual-band and multispectral lines short-pulse seeds in a fast flow CO2 laser amplifier, analyzing the effects of input pulse fluence, pulse width, and spectral line composition on amplification efficiency. Compared to single-line 10P(20) amplification, the extraction efficiency of 10.6-µm and 9.6-µm dual-band two-line 15-ns pulses is improved by approximately 70%, surpassing that of 10.6-µm single-band four-line amplification. This study is of great significance for the efficient amplification of short-pulse CO2 lasers and the generation of extreme ultraviolet light from laser produced plasma.