Aiming at the inconsistency between the results of precipitation water resources assessment and annual scale water resources assessment, a new evaluation method of precipitation water resource is proposed in this paper by splitting the runoff components such as antecedent precipitation recession and annual scale base flow, in order to realize the refined evaluation of precipitation water resources. The analysis results in the typical basin of Chaobai River show that the method has good adaptability, significantly improve the pertinence and accuracy of the assessment results of precipitation water resources, and fulfills the needs of water resources assessment with multiple temporal and spatial scale coordination.