Typical　high　efficiency　video　coding　(HEVC)　encoders　use　rate-distortion　optimization　(RDO)　to　select　the　best　coding　parameters　among　numerous　candidates　to　achieve　high　compression　efficiency.　However,　calculating　the　rate-distortion　cost　for　each　coding　parameter　entails　high　computational　complexity,　which　limits　the　real-time　application　of　these　encoders.　In　this　study,　time-consuming　processes,　such　as　reconstruction　and　entropy　coding,　are　removed　during　the　calculation　of　the　full　RD　cost　to　reduce　the　complexity　of　the　encoder.　Instead,　RD　costs　are　estimated　for　transformed　coefficients　by　using　the　RD　cost　model,　which　is　made　up　of　a　distortion　model　based　on　the　distribution　features　of　the　transformed　coefficients,　a　residual　rate　model　based　on　the　characteristics　of　the　entropy　coding　module,　and　a　simple　and　convenient　header　rate　model.　In　addition　to　the　full　RD　cost,　typical　encoders　also　adopt　fast　RD　cost,　which　is　composed　of　the　sum　of　absolute　transform　differences　(SATD)　and　header　bit.　To　further　reduce　encoding　complexity,　three　simplified　SATD　(SSATD)　methods　based　on　DCT　and　DST　features　are　proposed　to　serve　as　substitutes　for　SATD　in　fast　RD　cost,　thus　reducing　the　computational　complexity　of　SATD　calculation.　Experimental　results　show　that　35.53%,　14.57%,　and　14.37%　of　coding　time　can　be　reduced　respectively　under　AI,　RA　and　LDP　configurations　with　negligible　RD　performance　loss　when　applying　the　proposed　RD　cost　model　and　the　SSATD　methods　to　HM16.7.　Besides,　the　proposed　methods　also　can　reduce　4%-9%　of　encoding　time　and　exhibits　the　same　RD　performance　as　state-of-the-art　methods.