The　geosynthetic　reinforcement　layer　techniques　have　been　widely　used　nowadays　in　the　foundations　and　embankments.　During　the　course　of　the　design　of　the　geosynthetic　reinforcement　layer,　the　confirmation　of　the　stress　diffusion　angle　is　very　important,　but　no　good　methods　to　calculate　the　stress　diffusion　angle.　This　paper　presents　the　method　of　calculating　the　stress　diffusion　angle　of　geosynthetic　reinforcement　layer　based　on　additional　stress　back-analysis;　and　then　using　this　method,　researching　the　setup　parameters　of　geosynthetic　reinforcement　layer　(the　thickness　of　geosynthetic　reinforcement　layer,　the　composite　modulus　of　compressibility　of　geosynthetic　reinforcement　layer)　and　geosynthetic　factors　(such　as　reinforcing　space,　reinforcing　width,　total　numbers　of　geosynthetic　reinforcement　layer　etc.)　on　the　distribution　rule　of　the　stress　diffusion　angle;　the　results　indicate　that　the　range　of　the　stress　diffusion　angle　is　45-60°;　further　more　the　setup　parameters　of　geosynthetic　reinforcement　layer　and　geosynthetic　factors　existing　optimum　values,　corresponding　on　these　values,　the　stress　diffusion　angle　reaches　the　max.　Finally,　using　orthogonal　tests　and　regression　methods,　many　effect　factors　on　the　stress　diffusion　angle　are　analyzed,　the　results　indicate　reinforcing　space　has　a　most　effect　on　the　stress　diffusion　angle　of　the　geosynthetic　reinforcement　layer,　secondly　the　embedded　depth　of　the　first　geosynthetic,　however,　reinforcing　width　has　a　slight　effect　on　the　stress　diffusion　angle.