Abstract: The spacecraft integration precision measurement is based on a cubic prism fixed on the equipment. Because of the limitation of the assembly position on the spacecraft, the use of the electronic theodolite and the current precision measuring software can not fully cover the angles of the three normals of the mutually perpendicular planes of the cubic prism relative to the spacecraft coordinate system. This paper proposes a method of replacing a cubic prism with a 135° irregular prism and a vector calculation method for the irregular prism, to deal with the vectors of three normals of the three mutually perpendicular planes of the 135° irregular prism relative to the spacecraft coordinate system. This method was tested, verified and applied in the follow-upspacecraft precision measurement.