Abstract: Brazil's Belo Monte hydropower was sent out through the two ±800kV UHVDC transmission projects. The main purpose of the master control (MC) safety control system, installed for this huge system, is to realize the action of the stability device and the execution of the coordinated control instructions between two large hydropower stations, two UHVDCs and the nearby AC system, to ensure the maximum transmission of hydropower in northern Brazil and the safe and stable operation of the power grid. This article deeply analyzes the design principles, coverage and action sequence of the MC system, and according to the power transmission requirements with the original stability control system of the Belo Monte Bipolar I project during the commissioning period, the targeted alternate isolation measures and the optimization commissioning plans are proposed to ensure all the stability control functions and the interface equipment of the MC system to be tested. The commissioning plan was implemented on the site. The action results were in line with expectations, and the conditions for operation were met.