By employing the self-imaging properties of multimode interference, a 1×3 optical switch with mono-compound modulation region is presented. When at different switching states, a multiple relation exists in the acquired modulation magnitudes in the connecting arms, and therefore, the phase shifters in the arms can compose a mono-compound modulation region (MCMR). By contimuously changing the refractive indices in the MCMR, light incident from any input port can be switched alternatively to arbitrary output port. Based on Al0.07Ga0.93As/GaAs ridge structure, the proposed device was optimized and theoretically demonstrated by employing BEAMPROP software. The extinction ratio is as high as 30.3 dB. The crosstalk between adjacent channels is less than -29.8 dB. In a wide spectral range (～50 nm) around 1.31 μm wavelength, the crosstalk keeps lower than -20 dB. This device may greatly simplify the modulating scheme of traditionaly multichannel switch and relax the complextity of external controlling.