The invention discloses an adaptive region spare power automatic switching system and method based on a composite power supply mode. According to the invention, the operation mode of a current composite power grid can be automatically identified, fault analysis and positioning are performed through collecting analog signals and protection action signals of a substation where a spare power automatic switching device is located in real time, an optimal power switching strategy is provided by combining on-site spare power automatic switching and wide-area spare power automatic switching at the same time, an electrical control command is outputted quickly to isolate the fault, and protection control is performed on the power grid. The response speed of the system is high, the power supply reliability is improved, the power supply quality is improved, and the operation efficiency of the power grid is improved, thereby improving the reliability of the power grid, shortening the power failure time caused by the fault, eliminating the structural safety risk from the source, realizing an automatic switching condition of the composite power supply system, enabling the power supply reliability of the power grid to reach the international advanced level, and having good application prospects.