Sudden expansion channel is the basic geometric structure of hydraulic components, the local pressure loss in this kind of geometric structure has always been treated as a constant value among standard textbooks. Based on overall balance of energy and momentum, the local pressure loss has been decomposed into 5 parts, namely the approximate theoretical value, the pressure loss caused by distortion of velocity in the upstream channel, the pressure loss caused by actual wall friction in the upstream channel, the pressure loss caused by actual wall friction in the downstream channel and the pressure loss caused by the nonuniformity of pressure at the expansion plane. By means of the CFD simulation approach, the local pressure loss in sudden expansion has been calculated among a range of Reynolds numbers. The research results show that, there is a critical Reynolds number Recr, when the actual Reynolds number Re is smaller than the Recr, the local pressure loss coefficient is inversely proportional to the Re; for low-Re cases, the pressure loss caused by the nonuniformity of pressure at the expansion plane is the major part of the local pressure loss; however, for high-Re cases, the approximate theoretical value and the pressure loss caused by actual wall friction in the downstream channel are the majors part.The theoretical analysis method proposed by this paper can be used for reference to the structural optimization of the sudden expansion channel among the hydraulic components.