Aiming to address the issues of current overload and reversal risks in traditional VF (constant voltage-frequency ratio control) and I/F (constant current-frequency ratio control) starting methods for sensorless startup control of three-stage synchronous generators, this paper proposes a collaborative control strategy integrating rotor position estimation based on pulse voltage injection with I/F startup. First, coarse positioning with an initial rotor position error of less than 30° is achieved by injecting 12 directional pulse voltage sets in the dq-axis coordinate system and detecting the current response. Subsequently, the estimated position is utilized as the initial phase angle for the I/F starting algorithm, effectively mitigating reversal risks. Furthermore, a startup state detection mechanism for three-stage synchronous generators is proposed based on active-reactive power analysis. Simulation results demonstrate that the proposed method successfully starts the synchronous generator while avoiding reversal and enables real-time detection of startup success, exhibiting strong practical value.