In view of the limitation of the simplified model,this paper proposes a modeling method for photoelectric auto-collimating system based on vector operation.With the theory of vector operation,arbitrary point imaging of the reticle is analyzed in that method.The mathematical model is derived from the relation of the reticle point coordinates,the reflector space angle and the image point coordinates.With its special case,this paper puts forward an ideal auto-collimating system.Finally,principle error of the simplified model and the corresponding relationship between image trace and reflector space angle are discussed and also simulation and laser auto-collimator experiment results validate the proposed model.The presented model has some practical significance in the design,error analysis and compensation of photoelectric auto-collimating system with large range and high precision.