With the aim to improve the unified performance of the first soft X-ray spectromicroscopic beamline built in Shanghai Synchrotron Radiation Facility(SSRF), This paper analyzes the main factors that effect the light spot transversal transfer of the spectromicroscopic beamline during wavelength scanning by a variable-included-angle plane grating monochromator. It deduces the relation between various factors and light spot transversal transfer, and solves the error distribution of the system to ensure the performance of the beamline. A test system for the repeatability of light spot transversal transfer is built by auto collimation principle and the off-line testing of the light spot transversal transfer is finished by using this test system. The result shows that the repeatability is 0.67″, which satisfies the technical requirement of 1″. The unified performance for the beamline is tested after it is assembled. Obtained results meet the requirements of design and application, and prove the effectiveness of error analysis and testing for the light spot transversal transfer.