A mathematical model for pressure transient analysis in polymer-flooding system was established, in which the shear thinning effect of non-Newtonian polymer solution is considered. The governing equations were discretized by finite volume method with a hybrid grid consisting of Cartesian meshes with radial refinement around well positions. The shut-in pressures from numerical solution were compared with those obtained with the superposition principle method. It shows that shut-in bottom-hole pressure computed with superposition principle method is much lower than the numerical result for polymer-flooding system, which indicating that the superposition principle cannot be used for pressure transient analysis in polymer-flooding system. Shut-in pressure data of an injection well in a polymer-flooding system was interpreted. It is found that the permeability obtained with superposition principle method is very small, which shows that the superposition principle method cannot be used to solve the nonlinear model for pressure transient analysis in polymer-flooding system. The numerical method is more suitable for polymer-flooding system. The conclusions are applicable to similar nonlinear problems in pressure transient analysis.