Double-winding AC-DC Generator are being widely used in self-contained electrical power systems for their obvious virtues. On their stator, two separate windings are distributed, one three-phase winding delivers AC power and the other twelve-phase winding, termed DC windings, are connected to the bridge rectifier to supply DC power. Unlike ordinary three-phase machines, there are four layers winding in each stator slot of the Double-winding AC-DC Generator and the total gap magnetic potential produced by the twelve-phase winding is more complex than 3-phase machines. By analyzing the leakage magnetic flux and the magnetic potential, relevant formulas are derived to calculate the leakage reactance of the machines. All the stator leakage reactance of a realistic Double-winding AC-DC Generator was calculated through the methods presented and the simulation of a sudden shortcut test of the machine was carried out with the parameters calculated. The simulation and test results agree quite well, and show that the formulas presented are correct.