Abstract: This paper uses the Galileo orbit parameter for simulating the Galileo satellite position and DOP,compares the visible satellite number and DOP between GPS and Galileo system,at same time and same place.The result indicates that Galileo constellation is better than GPS.And simulates GPS and Galileo combined constellation,more visible satellite brings diminished DOP, in contrast with single system,so that improves the position precision,and also broaden the scope of position due to limited elevation.