Abstract: To address the apparent decline of liquid productivity index during polymer flooding in some offshore heavy oil wells, a sophisticated mathematical model to predict productivity behavior in multi-layered reservoirs is established, then the main controlling factors on the variations of liquid productivity index of polymer flooding are studied. The results show that the productivity index after conversion into polymer flooding undergoes 4 phases — rise, decline, stabilization and rise again; the largest drop of the dimensionless liquid productivity index is significantly influenced by polymer viscosity, polymer injection timing and permeability ratio, and the trough timing is highly impacted by crude oil viscosity, permeability ratio, relative thickness of low-permeability zone and absorption capacity. Based on which formulas to calculate the largest drop of dimensionless liquid productivity index and trough timing are proposed. Under the typical polymer flooding conditions in offshore oilfields, the average largest drop of liquid productivity index is 53.1%, and the average cumulative injection pore volume corresponding to the trough timing is 0.250 PV. The research results are of guiding significance for the liquid productivity index prediction, program design and adjustment for polymer flooding.