Inertia constant is widely used to evaluate the inertia capability for wind turbines (WTs). However, the method fails to reflect the dynamic characteristics of the inertia and neglects the impacts of controllers. For this issue, this article establishes an inertia function to analyze the dynamic inertia for type-3 WTs. Firstly, the consistency of the power transfer mechanisms between the type-3 WT and synchronous generator (SG) is discussed. Then, a new approach to construct the inertia function is offered according to the phase motion model of SG. On this basis, the impacts of the controllers and operating points on the inertia are explored, and the critical factors are revealed. The analysis results help to understand and optimize the inertia capability of the type-3 WT. Moreover, compared with the conventional inertia constant, the presented inertia function can better reveal the dynamic nature and the influence rule of inertia. Finally, the analysis is validated by IEEE 3-bus test system simulation.