The TianQin Station has applied a multimode fiber (MMF) coupled superconducting single photon detector (SNSPD) in the space target laser ranging system. The coupling manner of the space light and MMF is used in the receiving system and it can affect the number of echo photons and ranging accuracy, especially in lunar laser ranging, high orbit satellite laser ranging and space debris tracking. When there is a pointing deviation angle from the telescope to the measurement target, it will directly affect the coupling efficiency and further affect the entire system efficiency. The paper analyzes the influence of the entire optical transmission process from the 1.2m telescope receiving system to the MMF on the optical fiber coupling efficiency. It mainly includes detailed calculations of the aberration, the spot properties and reflection. The variation of MMF coupling efficiency corresponding to different telescope deviation angles for 200um multimode fiber is given.