Spatial alignment is the first problem that needs to be solved in multi-plane bearings-only passive localization. Therefore, a suitable spatial alignment method is proposed for this application background. The proposed method selects the fusion plane's geographic coordinate system at initial time as the fusion center coordinate system. Firstly, the bearing got in each plane's body coordinate is converted to the one in each plane's geographic coordinate system, which is further converted to the coordinate system which is parallel to the geographic coordinate system of the fusion center and has its origin located at the centroid of the plane. Besides, the analytical expression is deduced for the bearing converted to this coordinate system. Simulation analysis verifies the influence of factors such as the target's direction of arrival, the attitude angle of the plane and the distance between the fusion center and the plane, which yields some quantitative conclusions.