Transmission properties of light through a double-slit compound grating with different slit widths are investigated with the finite-difference time-domain (FDTD) method. The results show that phase resonance (PR) splitting can be achieved with the slit widths modulation, and it can be simultaneously modulated at multiple frequencies. All modes have sharp PR splittings, and the values of some valleys are almost zero. Secondly, based on the electromagnetic field distribution, Fabry-Perot resonant and phase resonant mechanism, the physical origins of the observations have been qualitatively described and explained. In addition, we can realize the control of light spreading through any slit in two slits, and also achieve the design of bandgap by splitting valley. These results can be used to design optical channel selector, frequency selector, filter and optical switch. Compared with the conventional compound gratings, this double-slit grating with different slit widths has obvious advantages, such as simple structure, single material, and easy realization, design and analysis.