Abstract: In order to explore the function mechanism of Ca²⁺ and Mg²⁺ on weak polymer gel displacement, aiming at geological characteristics and fluid property of one reservoir in Bohai Oilfield, this paper studied on the influence of Ca²⁺ and Mg²⁺ on displacement effect of Cr³⁺ polymer gel through evaluating increasing viscosity, shear resistance, temperature tolerance, viscoelasticity, seepage characteristic and displacement effect, with taking viscosity, viscoelasticity, resistance factor, residul resistance factor and oil recovery as evaluation factor. The results show that, compared with injection water, although softened water has higher salinity, it almost doesn't contain Ca²⁺ and Mg²⁺. Polymer molecular chains of Cr³⁺ polymer gel prepared by softened water suffer less influence of Ca²⁺ and Mg²⁺ divalent catio, and have bigger polymer molecular aggregation size, its increasing viscosity, shear resistance, temperature tolerance, viscoelasticity, seepage characteristic and displacement effect are all stronger than Cr³⁺ polymer gel prepared by injection water. Using softening process for offshore oilfield injection water, which means eliminating Ca²⁺ and Mg²⁺, is good for improving displacement effect of Cr³⁺ polymer gel, and furtherly enhancing oil recovery.