Promoted by the extensive integration of distributed power resources, grid-connected inverters are progressively expected to function as grid-forming units. This requires them to not only regulate grid voltage and frequency but also provide inertia to compensate for the decline in traditional synchronous generation. Current studies on grid-forming inverters (GFMIs) have primarily focused on small-signal stability. However, the transient stability during voltage sags has not been adequately addressed. In this paper, the transient stability of GFMI is analyzed with the phase portrait, and an acceleration power control scheme is first proposed, which effectively constrains frequency deviations during both fault occurrence and clearance processes without requiring the estimation of voltage sag depth or line impedance. Remarkably, the proposed method ensures maintained synchronization stability of GFMIs even under severe grid faults. Finally, a 500W experimental platform is constructed in our lab to verify the effectiveness of the transient stability enhancement method.