The invention discloses an overbank flow floodplain and channel excess flow computing method. According to the method, a computing formula of floodplain and main channel excess flow is built, and the floodplain and channel excess flow ratio in the computing formula is closely related to the water depth ratio, the width ratio and the roughness factor ratio. In order to determine a mutual relation, a compound section transverse distribution flow mathematical model is adopted for computing the floodplain and channel excess flow under different conditions, the relational expression between the floodplain and channel excess flow ratio and the water depth ratio, the relational expression between the floodplain and channel excess flow ratio and the width ratio, and the relational expression between the floodplain and channel excess flow ratio and the roughness factor ratio are obtained through analysis, and finally the overbank flow floodplain and channel excess flow is comprehensively obtained through the computing method. According to the computing method, when fluvial cross section conditions are certain, the floodplain and channel excess flow can be rapidly obtained according to the forecast cross section flow, and a basis is provided for planning and utilization of floodplain and water resources.