The small fault block edge water reservoirs are characterized by small volume, narrow oil bearing area. In early development stage，if geological knowledge is not clear, the accurate evaluation of the size of edge water is not carried out, it will reduce development efficiency. Authors analyze the geology and production characteristic ofWest 58- 8 fault block reservoir, calculate the size of the water aquifer by use ofmaterial balance equation and evaluate the edge water energy by use of numerical simulation. The calculation results showthat twomethods （material balance equation and numerical simulation） are quite consistent. The model is accurate and reasonable.