The four-parameter model is often used in astronomical fixation,but the coordinate scale and change of the angle between the coordinate axis are not taken into consideration.Therefore,the four-parameter model is only a simplified model for coordinate transformation of astronomical fixation.In order to verify the applicability of the model,and improve the precision when dealing with the transition between the tangential coordinate and the CCD image coordinate,a six-parameter model was built up based on the four-parameter model.Contrastive analysis was made to the two models in the aspects of positioning accuracy and tilt angle compensation.The analysis result to data obtained from the experiments demonstrated that the two models have basically the same precision,thus the applicability of the four-parameter model was proved.