Mode　superposition　approach　is　one　of　common　dynamic　analysis　methods,　in　which　the　number　of　modes　involved　in　the　approach　has　significant　effects　on　the　calculation　results.　The　mathematical　backgrounds　and　physical　meanings　of　participation　mass　ratio　method　and　displacement　control　method　are　revealed　in　this　paper　and　their　deficiencies　are　indicated.　Based　on　the　errors　of　modal　truncation,　a　new　measure　called　norm　method　to　determinate　reasonable　mode　numbers　is　put　forward,　which　has　a　rigorous　theoretical　base　and　is　suitable　to　any　form　of　dynamic　load.　A　numerical　example　shows　that　the　proposed　method　is　more　reasonable　and　effective,　while　the　mode　number　determined　by　the　participation　mass　ratio　method　is　fewer　and　that　by　the　displacement　control　method　is　too　conservative.　©　2017,　Editorial　Department　of　CJAM.　All　right　reserved.