Abstract: Solving methods of rational well spacing desity and critical well spacing density of infill producers in developed oil field are derived by using the basic economic principle of revenue and expenditure balancing and on base of analysis of the relationship between well spacing desity and ultimate oil recovery． This method is used to estimate and analyze each block in F oilfield under different oil prices． The central and east blocks are the favorable areas for infill drilling in F oilfield． These study results provide basis for rational and highly effective development of F oilfield．