The invention discloses a method and system for acquiring power receiving capacity of a regional power grid and belongs to the technical field of electric power systems. The method comprises the stepsof determining the most serious fault and the weakest line of a regional power grid system: calculating the ratio of the power on the weakest line to the maximum power of the line after in the initial state of the regional power grid, the most serious fault occurs, causing transfer of power flow; determining the maximum power receiving capacity of the regional power grid to be m times of the initial receiving power; 1.1, adjusting the power flow of the regional power grid; 1.2, determining the most serious fault and the weakest line; 1.3, performing time domain simulation on the fault and checking whether the regional power grid system is unstable or not; and 1.4, acquiring the maximum power receiving capacity of the regional system. According to the method and the system in the invention, the ratio of a power flow transfer value caused on other lines after the fault of a certain line of the regional system power transmission section in different states to the maximum power transmission power of the line is given, a line fragile factor is obtained, and the most serious fault and the weakest line of the system can be determined according to the size of the line fragile factor.