Abstract: Systematic monitoring and testing are conducted for the process of polymer injection for offshore oilfields, in order to ensure the quality of “one-pump-multi-well” polymer injection for offshore oilfields. Through the adjustment test, the mechanism of inter-well flow caused by the inconsistency between “well pump pressure” and “inter-well pressure” is proved. A method for judgment of inter-well flow and the correlation between the direction of inter-well flow and the number of one-pump wells are presented in this paper. According to the mechanism of “inter-well flow”, measures for preventing inter-well flow are given. Field test shows that “inter-well flow” occurs during polymer injection. On the basis of the characteristics of polymer injection technology for “one-pump-multi-well” in offshore oilfields, the occurrence of “inter-well flow” may lead to the change of injection ways of polymer injection wells, i.e. the change from “continuous polymer injection” into “water alternating polymer injection”. As a result, the polymer injection quality may be affected seriously. It is suggested that the number of one-pump wells be less than three. It is necessary to optimize the pump-well combination while conducting technology design.