Abstract: The guide telescope ,as an integral part of the guide system of the 1m new vacuum solar telescope (NVST)，its stability of imaging is essential. It is necessary to be concerned that the deformation of the telescope structure caused by environmental thermal load may cause thermal defocusing and affect the image quality of the guide telescope. The temperature distribution of guide telescope at -5 ℃, 20 ℃ and 35 ℃ were analyzed using finite element method, and then the axial deformation of guide telescope caused by temperature field was calculated. The maximum defocusing amount is 6.45 m, and the results show that the max change of distance between the primary mirror and the target surface is far less than the depth of focus（35 m） ,which provides a basis to analyze the non-thermal design of guide telescope of NVST.