This letter proposes a method for accurately calculating RF power at a standing wave (SW) transmission line system. It uses two directional couplers and a low-level radio frequency(LLRF) control system. The RF power is calculated from the proposed equations using amplitude and phase measurements of the coupling signals from the two directional couplers. The method is demonstrated adopting two directional couplers with directivity 26 dB and 28 dB respectively at 162.5 MHz over a whole wavelength range. The average power errors observed when using a single directional coupler are about 6.6% and 5.8% respectively, and the calculated power error can be reduced to about 1.0% through the proposed method.