Introduction:　The　unilateral　external　fixator　has　become　a　quick　and　easy　application　for　fracture　stabilization　of　the　extremities;　the　main　value　for　evaluation　of　mechanical　stability　of　the　external　fixator　is　stiffness.　The　stiffness　property　of　the　external　fixator　affects　the　local　biomechanical　environment　of　fractured　bone.　Methods:　In　this　study,　a　theoretical　model　with　changing　Young＇s　modulus　of　the　callus　is　established　by　using　the　Castigliano＇s　theory,　investigating　compression　stiffness,　torsional　stiffness　and　bending　stiffness　of　the　fixator-bone　system　during　the　healing　process.　The　effects　of　pin　deviation　angle　on　three　stiffness　methods　are　also　investigated.　In　addition,　finite　element　simulation　is　discussed　regarding　the　stress　distribution　between　the　fixator　and　bone.　Results:　The　results　reveal　the　three　stiffness　evaluation　methods　are　similar　for　the　fixator-bone　system.　Finite　element　simulation　shows　that　with　increased　healing　time,　the　transmission　of　the　load　between　the　fixator　and　bone　are　different.　In　addition,　the　finite　element　analyses　verify　the　conclusions　obtained　from　the　theoretical　model.　Conclusions:　This　work　helps　orthopedic　doctors　to　monitor　the　progression　of　fracture　healing　and　determine　the　appropriate　time　for　removal　of　a　fixation　device　and　provide　important　theoretical　methodology.