The　＇S＇　shaped　test　piece　is　used　to　comprehensively　detect　the　machining　accuracy　of　multi-axis　machine　tools.　However,　the　existence　of　theoretical　cutting　error　of　＇S＇　shaped　test　piece　leads　to　the　deviation　of　accuracy　detection　for　multi-axis　machine　tools.　Therefore,　an　effective　method　to　reduce　the　theoretical　cutting　error　is　proposed.　Firstly,　the　uniform　double　cubic　B-splines　surface　model　is　applied　to　establish　the　＇S＇　shaped　test　piece　model.　Furthermore,　the　distribution　of　twist　angle　at　the　selected　three　section　lines　is　obtained.　Meanwhile,　the　influence　of　twist　angle　on　theoretical　cutting　error　of　＇S＇　shaped　test　pieces　is　analyzed.　Then,　the　optimized　single-point　offset　(OSPO)　method　is　proposed　based　on　the　single-point　offset　(SPO)　method.　Moreover,　the　tool　path　of　＇S＇　shaped　test　piece　is　generated　by　CAD/CAM　software　based　on　the　OSPO　method.　Finally,　the　effectiveness　of　the　proposed　method　is　verified　by　the　experiment.　Experiment　results　show　that　the　average　theoretical　cutting　error　of　＇S＇　shaped　test　pieces　based　on　the　OSPO　method　is　reduced　about　50.1%　than　the　SPO　method.　In　addition,　the　vast　majority　of　theoretical　cutting　errors　based　on　the　OSPO　method　are　approximately　less　than　0.005　mm,　which　can　be　negligible.　It　is　therefore　reasonable　to　conclude　that　the　proposed　method　here　can　avoid　the　influence　of　theoretical　cutting　error　on　the　accuracy　detection　of　multi-axis　machine　tools　efficiently.　And　its　basic　idea　can　be　applied　to　other　type　of　test　pieces.　©　2020　Journal　of　Mechanical　Engineering.