Initial rotor position detection is one of the key techniques for large synchronous motor frequency starting. Precise determination of the initial rotor position can realize maximum-torque starting. The real time digital simulator（RTDS） is used as the platform for investigating the converter starting control theory of large synchronous motors. Accurate initial position determination based on the adaptive magnetic flux regulator is proposed by combining the mathematical model for synchronous motor rotor position with the adaptive magnetic flux regulator model. The algorithm is verified by loop simulation experiments， 6 kV/500 kW motor dynamic tests， and Panjiakou pumped storage unit starting tests. Experimental results show that the method can quickly and accurately measure the initial rotor position.