Abstract: Firstly, the relationship between transient stability and synchronous torque of generator was analyzed, the influence of Reactive Current Compensation(RCC) and excitation ceiling voltage ratio on transient stability was transformed into that on synchronous torque for mathematical derivation. Then, the mechanism of RCC and excitation ceiling voltage ratio affecting synchronous torque of generator was deduced and calculated by an example, followed by the simulation of the single-machine system, two-machine system and actual system. The results of calculation and simulation show that minus RCC coefficient provided minus synchronous torque and plus RCC coefficient provides plus synchronous torque for hydro-generator under heavy load, and the synchronous torque provided by RCC changes in parabola for hydro-generator under light load. For turbine-generator under all loads, minus RCC coefficient provided plus synchronous torque and plus RCC coefficient provides minus synchronous torque. Since the excitation voltage reached the upper limit in case of short circuit fault, it has nothing to do with the RCC coefficient, so the RCC coefficient would not affect the transient stability of the generator. Different from the general understanding, it can be found that the larger the excitation ceiling voltage, i.e. E_FDMAX is, the better the transient power angle stability is, but the impact on the transient power angle stability of the units is small. During the short-circuit fault, the absolute value change of E_FDMAX has little effect on the State Grid Co., Ltd. Science and Technology Research Program: Research and Application of Adaptive Optimization and Oscillation Suppression of Distributed Condenser in High Proportion New Energy Areas (5100-202134015A-0-0-00). transient potential of q-axis, i.e. E'_q0 and the synchronous torque, i.e. K_S. After the short-circuit fault disappears, the influence increases, but it is still small. After the short-circuit fault disappears, the increase of E_FDMAX has obvious effect on the reactive power output of generator and the voltage of high side bus, which is the main function of increasing E_FDMAX.