Abstract: According to the positioning principle of double-star position system,firstly,we give the position measurement equations,based on the position error model,at the same time deduce the position error model of double-star position system under the mechanism of difference position.Then,we give the position error model generated by altitude error and ephemeris error.The conclusion shows that the large ephemeris error is equivalent to one minor range error after difference.At last,we simulate synthetically the position error of this system and show the error distributed characteristics of double-star position system.