Abstract: With the continuous operation of ultra-high voltage alternating and direct current power transmission and transformation projects in Inner Mongolia autonomous region, Inner Mongolia power grid has become a ultra-high voltage alternating current and direct current hybrid system. However, the development of alternating current and direct current projects is not balanced, the characteristics of "strong direct current weak alternating current" is obvious, the interaction among AC/DC, multiple DC, sending and receiving end is increasingly significant, the operation complexity of power grid increases rapidly. In this case, the matching thermal power unit at the sending end connected to the ultra-high voltage alternating current and direct current hybrid power grid will inevitably affect the safe and stable operation of ultra-high voltage power grid, the reliability of power supply is difficult to be guaranteed. In this paper, the sending end regional power grid of Xilinhaote to Jiangsu Taizhou ultra-high voltage direct current transmission project (hereinafter referred to as "XITAI DC") is taken as the research object. Using PSASP (Power System Analysis Software Package) simulation Software, the simulation calculation of power flow distribution, voltage level and transient process is carried out, which is after the operation of the the matching thermal power unit of the sending end in XITAI DC. The influence on the safe and stable operation of the power grid after the high-power supporting power supply is connected to the ultra-high voltage alternating current and direct current hybrid power grid is deeply analyzed, and put forward the corresponding effective prevention and control measures.