In virtue of geometric projection, the path difference equation can be obtained which is associated with geometric parameter. Thus, the DF analytic solution for long baseline can be derived based on the path difference and cut angle between two stations. On this basis, a DF formula for two stations, that the format is simple and veracity is also good, can be obtained by approximate reduction. The analysis result shows that using cut angle between two stations can effectively improve the veracity of existence DF solution of two stations.