Synchronizing out of phase may sometimes occur to a turbine generator due to the misoperation or failure of a synchronizing controller. There could be a large impulse current in stator windings during out of phase synchronizing. The impulse current could seriously damage the windings insulation, and the magnetic flux density might be easily supersaturated at the rotor pole surface, leading to stronger stress on the rotor tooth and damage the turbine generator. Aiming at studying the electromagnetic field when synchronizing out of phase occurs, a mathematical model of the transient electromagnetic field with field-circuit-grid coupled is established by taking a 150 MW air-cooled turbine generator for consideration. Impulse current in stator winding and supersaturation at the rotor pole surface are calculated respectively in different angles between the vector of the turbine generator electromotive force and voltage vector of power grid. According to the largest impulse current in stator windings at the 180 degrees out-of-phase synchronization, the influence on the magnetic field distribution and saturation at the rotor pole surface of turbine generator stator resistance and the end region leakage inductances that vary in a certain range is studied. The results could provide theoretical basis for researching of electromagnetic field at out-of-phase synchronization of large turbine generator in the end zone.