In order to achieve the optimal back passivation effect of monocrystal PERC cell and find out the optimal process conditions, the main process parameters were optimized by orthogonal experimental method. After being passivated by back passivation, the thickness and refractive index of the film and the conversion efficiency of the cell were measured. The results of orthogonal experiments show that the optimal process conditions of back passivation are 0.25 mbar chamber pressure, 360 ℃ temperature setting, 2.8 NH3/SiH4 ratio, 1 500 W microwave power. Under this conditions, the average test efficiency of the cell reaches 22.35%, and the electrical performance indexes such as Voc and Isc reach a high level.