The short-circuit capacities of buses are key parameters required for the operation and control of power systems. On-line measurement of short-circuit capacity is an effective method to achieve the observability and controllability of power grids. This paper proposes an on-line measurement method to estimate the short-circuit capacity of the grid bus according to the bus voltage fluctuation triggered by switching the parallel capacitors. Firstly, based on the circuit model of substation and the substitution and superposition theorem of the linear circuit, an accurate calculation method for measuring the bus short-circuit capacity is derived. It reveals that the phase-angle differences of the voltage vector caused by the capacitor switching is a crucial factor affecting the measurement accuracy, which is an important improvement compared with the approximate formula of previous studies. The proposed measurement method has high precision. The proposed iterative calculation method for the voltage vector phase difference is more convenient and reliable than the direct measurement. The effectiveness of the proposed on-line measurement method is tested on a detailed substation bus model on Matlab/Simulink. According to the tests in various situations, the simulation results show that measurement error of the proposed method is less than 1%.