Abstract: On accound of formation flowback in wells such as polymer residue return and sandback which resulted in unable to separated-inject normally in oilfield after polymer flooding, we have carried out flowback-prevention separated injection research in injection well.we have invented the tools―shear filter and one-way water injection allocator and strings that can solve the problem in separated injecting wells. The shear filter have two cup packers and a slotted liner, they can set up a tie-back liner in injection interval, and prevent the polymer flowback and sand return into the separate injection string. This technology have been used in many injection wells, the result showed the separated injection valid period doubled, and initially resolved the difficulty.