Abstract: Grid-forming doubly fed induction generators (GFM-DFIGs) exhibit active supporting capacity similar to the synchronous generator (SG), but also faces similar transient stability problems as SG. However, the current limiting of the rotor-side control and the strong coupling between the rotor current and the stator current make the transient modeling and analysis of GFM-DFIG more difficult. In this paper, the model of GFM-DFIG is reduced, and the second-order model of GFM-DFIG for transient analysis is obtained. Considering the difference between virtual power angle and terminal power angle, the virtual power angle characteristic curve of GFM-DFIG under rotor current limiting is derived to achieve accurate characterization of dynamic characteristics of GFM-DFIG. By approximating the damping effect, an improved transient energy function method with damping is proposed. Based on the proposed second-order model, the evaluation indexes of the transient stability limit voltage (TSLV), the critical clear angle (CCA) and the critical clear time (CCT) are calculated. Quantitative analysis is carried out based on the above indexes, the influence law of grid impedance and control parameters on the transient stability is revealed, and guiding principles for GFM-DFIG parameters optimization are given. Finally, the simulation verifies the correctness of the theoretical analysis.