A theodolite is used to measure the change amount of the attitude of a heavy spaceborne deployable antenna during ground test before and after gravity unloading, the change is reversely preset to obtain the initial pose of the antenna under the stiffness condition of a mounting base, and gravity unloading is carried out to balance the gravity in order to eliminate the influences of the stiffness of the antenna mounting base on the gravity balance of the antenna, so the weightless environment condition of the on-orbit deployment is well simulated, and the deploying performance of the antenna is guaranteed. The ground test gravity balance method of the heavy spaceborne deployable antenna belongs to the technical field of space, and is provided to solve a problem that no effective on-orbit weightless environment condition simulation of the gravity unloading, caused by the influences of the stiffness of the mounting base on the heavy spaceborne deployable antenna during the ground test, finally affects the deploying performance of the antenna.