Navigation positioning accuracy is related with the number of satellite, although too much satellite can improve positioning accuracy, but this may cause huge computation. By studying the relationship between absolute value of determinant and geometric dilution of precision, an improved satellite selection algorithm was presented, which was applied to combined constellation system, and the positioning accuracy in different combination satellite number was analyzed. Simulation results indicate that improved satellite selection algorithm has low computational complexity, high positioning accuracy, and positioning error is very small by using least squares algorithm after 5 iterations, all of these can indicates the feasibility of improved algorithm.