To study the load safety margin of a power substation under maintenance or fault, a calculation model of the load capacity of the substation and a load control method considering the dynamic capacity of the equipment are proposed. First, a dynamic current-carrying capacity calculation model of the main equipment of the power substation is established according to the loading guide and temperature rise limitation. This helps obtain the dynamic capacity of the equipment. Second, the power supply capacities of the substation system and each busbar load point in its recent topology connection mode are determined by adopting the maximum flow method and considering the load transfer within and between the power substations. Then the load safety margin or overload amount are determined according to the current load level. Third, the corresponding load control method is taken according to the load situation, so as to ensure the power supply relying on the load priority when the power supply requirements are not met. The excess load is removed to guarantee the safe and stable operation of the system. Finally, the case study shows that the proposed method can put forward the corresponding load transfer or load removal strategies for various outages, and fully excavate the power supply capacity of the substation system and improve the power supply reliability at the same time as ensuring the safety of the main equipment. This work is supported by the National Natural Science Foundation of China-State Grid Joint Fund for Smart Grid (No. U1866603) and the Science and Technology Project of Guangzhou Power Supply Bureau “Research on Power Substation Operation and Maintenance Risk Assessment Method and Visual System” (No. GZHKJXM 20170083).