We design a silicon microring modulator based on mutual-mode coupling tuning by introducing an electrically tunable grating in the microring. By tuning the grating reflectivity that changes the mutual coupling strength, optical modulation is realized since transmission switches from a peak to a dip at the resonant wavelength with resonance-splitting in the ring. High modulation depth and low energy consumption can be achieved as sufficient grating reflectivity change can be obtained with low drive voltage. Simulations show that the proposed modulator can achieve a modulation depth of $\sim 13\mathrm{dB}$ at 1550 nm wavelength with energy consumption of $122.3\mathrm{fJ}/\text{bit}$.