Abstract: Double-star navigation position system adopts the mechanism of double-star position,and the basic position principle is three-sphere intersection measurement.Two observations are related to the distances between the two satellites and the survey station,and the third observation is related to the survey station's height,which is the critical factor that determines the system's position accuracy.Hence,we introduced the concept of similar ellipsoid,and deduced the accurate model of the third observation.Then,we established the position algorithm.The simulating experimental results show that this algorithm has a quick convergence rate and high accuracy of position.