For a centralized solar district heating system (CSDHS), an appropriate auxiliary heat source (AHS) is essential to improve the financial benefits. In order to find an economical AHS for the CSDHS in Tibetan Plateau, China, four typical cities (Nyingchi, Qamdo, Lhasa, and Shigatse) and three most commonly used AHSs (electric boiler, gas boiler and air source heat pump) are selected as the research objects in the current work. The levelized cost of heat (LCOH) is taken as the main evaluation index, and the software package, TRNSYS, is used to simulate and analyze the performance of the system. The selection of AHS and the corresponding solar fraction (SF) of the CSDHS are optimized. The results show that the electric boiler is suitable for areas with abundant solar irradiation, where the SF should be greater than 85%. The air source heat pump is found to be suitable for areas with ordinary or poor solar irradiation, where the SF is less than 50%. The change in maintenance cost has a significant influence on LCOH. Moreover, the higher the SF, the greater is the influence of maintenance cost on LCOH. In addition, when the air source heat pump is used for AHS, the non-guaranteed rate can be appropriately increased, which improves the overall economy of the system. The results can be used to effectively optimize the economic performance of a CSDHS, and provide reference for the design of an AHS and SF in different areas of the Tibetan Plateau, China.