The power equipment of a large system consists of three engines, which has two kinds of design scheme: cold standby and down-rating. The cold standby design scheme is that two engines run in power rating and the left one is reserved; after one failure, the cold reserved engine is switched. Yet the down-rating design scheme is that three engines work at the same time, once one fails, the other two run in power rating. On condition that each engine-s life distribution follows the different Weibull distributions, the reliability models were established for these two design schemes. Based on the life distributions of engines in power rating situation and down rating situation, the time-varying reliability curves of these two design schemes were given. Then according to the comparison with the two design scheme-s reliability curves with time varying, the better design scheme was chosen. At the end, the example shows that the method is reasonable.