Under extended-phase-shift (EPS) modulation, both the internal phase-shift angle and the external phase-shift angle will have large step changes when the dual-bridge series resonant converter (DBSRC) rapidly adjusts the transmission power. This will cause serious oscillations in the resonant current and resonant voltage of DBSRC and threaten the safety of the converter. This paper analyzes the transient process produced by sudden changes in phase shift angles under EPS modulation. Then a fast transient control method is proposed for suppressing the oscillation by utilizing both internal and external phase-shift angles, which controls the transient process within one switching cycle and achieves a smooth transition of transmission power. Besides, impact and oscillation on the resonant current and resonant voltage during transient processes are significantly reduced. Finally, the performance of the control method is verified based on experimental and simulation results.