There are only 2 or 3 satellites in the ＂Beidou＂ satellite navigation system, so it alone can not meet the demand of passive positioning algorithm. Three kinds of passive positioning algorithms about ＂Beidou＂ satellite navigation system are presented and compared： 2 satellites algorithm, 3 satellites 3 unknowns algorithm and 3 satellites 4 unknowns algorithm. The measurable parameters have been increased by adding height measurement and the unknowns have been decreased by modeling the receiver clock error with a 2 order polynomial. The availability of backup satellite of the ＂Beidou＂ navigation system is also studied. In addition, a technique named ＂quasi-differrential＂ correction scheme has been proposed. The positioning accuracy is improved by this technique. The experiment results show that all the three algorithms can achieve the accuracy of 100 m which can meet the common user＇s positioning demand.