Multi-GNSS(Global Navigation Satellite System) united positioning was simulated based on researches of high earth orbital positioning. On the weakest signal acquisition C/N₀ threshold, 15dBHz, feasibility of satellite beam main lobe and side lobe receiving was analyzed, and the dilution of precision values with different GNSS system united cases were also analyzed. Simulation results show that when the receiving signal’s C/N₀ of both main lobe and side lobe is larger than the threshold, 3 systems united or more can satisfy positioning. When the side lobe loss isn’t compensated, C/N₀ of receiving signal is weaker than threshold so that any case of multi-system united can’t realize positioning. Analysis of dilution of precision values shows that geometrical dilution of precision of single system and double systems united changes violently. Geometrical dilution of precision of 4 systems united decreases 16.93% than that of 3 systems united. On the condition of 3 systems united, the case of American GPS(Global Position System) united with Chinese BDS(BeiDou navigation satellite System) and European Galileo system(Galileo satellite navigation system) is best since its geometrical dilution of precision is smoother than others. Theory analysis and simulation results provide beneficial suggestions for researching of lunar explorer positioning and space borne GNSS multi-system receiver.