According to the one-dimension design theory of ejector, cooling the secondary flow before it enters into the mixing chamber can promote the eject efficiency and increase the eject coefficient, but flow loss is brought with the addition of precooler, so the performance evaluation of ejector is necessary. Looking on the eject system with precooler, the effect of precooler on the performance of system was analyzed through the onedimension theory, and the effects of the intensifying effect of precooling and the weakening effect of resistance on eject efficiency were analyzed emphatically. The research results indicate that: the precooler brings intensifying effect of precooling and weakening effect of resistance to eject system, and the cross section area and heat transfer area of the precooler are the dominating factors; cross section area of the precooler has a critical value, when the cross section area is bigger than the critical value, the eject performance improves with the increase of heat transfer area, otherwise it worsens; the eject performance of the equivalent pressure mixing scheme is better than that of the equivalent area mixing scheme, the eject coefficient of the former is 60% higher than the latter; precooling improves the eject performance effectively, especially for the equivalent area mixing scheme, the performance is improved by 35.5%.