An air-core compulsator is a special synchronous alternator which is made of nonferromagnetic material instead of traditional ferromagnetic material. The method of magnetic field calculation and analysis is different from that of the traditional machines. In this paper, an analytical method used to calculate the magnetic field of the air-core compulsator has been presented. The result calculated by a finite-element method based the ANSYS software is also presented. The difference between the magnetic field distribution of the air-core machine and the iron-core machine is compared and discussed.