As the extensive development of unconventional energy resource, hydraulic fracturing technology is developed and more widely used than before. Technology of forced fracture closure is an effective measure to guarantee high conductivity fractured reservoir. Nevertheless, reasonable choke size during backflow is always an enigma which confuses engineers in oil field. Big choke size may leads to backflow of proppants and small choke size may causes damages to reservoirs. Combining the principle of material balance with model of proppant back flow and considering characteristics of fracture during closure, the paper deduces the equation between well head pressure and reasonable choke size which shows fracture close time is the critical state and small choke size is necessary before fracture close and bigger choke size is reasonable to speed up the rate of backflow.