Abstract: Themostimportantthinginmannedlunarexplorationsissafety,whethertheprobecanfreereturnwhenfacingpowerfailureattheperiluneisanimportantcriterion.Withtheaimofdesigningcislunarfreereturntrajectory,tworequirementsshouldbesatisfied,thefirstistransfertrajectorydesignbetweentheearthandmoon,thesecondiswhethertheprobecanflybackafterlunarswing-by.Combinedwiththepreciseastronomicalephemeris(DE405),onetrajectoryindifferentmechanicalmodelswascomparedinordertodeterminewhatperturbationsshouldbeconsideredinthepreliminarytrajectorydesign.Then,almostallfactors(departuretime,transfertime,inclinationanglescombination,etc.)affectingthereturnperigeedistanceoftheprobewasanalyzed.Aunidirectionalsearchmethodwasproposed,whichcouldmaketheprobe′sreturnperigeedistancetendtothesettargetvaluegradually.Themethodissimpleandreliable,anditcansatisfyalltherequirementsofinitialtrajectorydesign.Theworkisofsomevaluetothecislunarfreereturntrajectorydesignofourmannedlunarexplorationsinthenearfuture.

Key words: freereturntrajectory, mechanicalmodels, returnperigeedistance, influencingfactors, inclinationanglerangearoundtheearth