In this paper, the effect of amplifier spacing on the performance of average-soliton transmission system using PSA as in-line amplifiers has been analyzed theoretically by computer simulation. Our simulation results show that when the ratio of amplifier spacing to dispersion distance is below 0.1, the performance of soliton system is well enough. But when it is above 0.1, the system performance is deteriorated quickly. The reason is that due to the phase sensitivity of its gain, PSA can cause soliton energy loss, thus result in the amplitude of soliton pulse descending significantly after long distance transmission. Raising PSA′s gain a little can compensate the loss, thus improve system performance. But the improvement is at cost of heightening the wing amplitude of soliton pulse.