The DFIG model based on mechanism modelling is complex, and does not consider the change of converter control strategy in voltage dip period. It is not suitable to analyze how DFIG voltage dip impacts power system transient stability. Usually, the during time of DFIG terminal voltage dip is short. The output of wind turbine generator is mostly affected by converter control strategy. Different DFIGs have different external characteristics. It is hard to get converter control strategy of DFIG, as it is a secret of a company. Mechanism modeling may face this trouble. Non-mechanism modeling constructs simplified DFIG model based on external characteristics. This paper contrasts testing data of two 1.5 MW DFIGs and analyzes output characteristics in voltage sag process. Simplified equivalent model is constructed both for voltage dip and recovery. The simplified model is constructed in PSASP software and compared to testing wind turbine. They have similar output curves. It shows that simple model is suitable for analyzing the effect of DFIG voltage dip on transient stability of power system.