Taking polystyrene powder as experimental material and dimensional accuracy as evaluation index, we study the effects of laser power, scanning pitch, layer thickness and scanning speed and their interactions on the forming precision of selective laser sintering parts. The mathematical model between process parameters and dimensional accuracy are achieved and the optimal process parameters are obtained by response surface methodology. The research results show that the dimensional deviation rate decreases with the increase of laser power and scanning speed, increases at first and then decreases with the increase of layer thickness, and increases with the increase of scanning pitch. The interaction between laser power and single layer thickness and the interaction between scanning pitch and layer thickness have a significant effect on the dimensional deviation rate. The maximum error between the predicted value of the response surface method and the actual value is less than 7%.