We propose a multichannel two-dimensional optical chemical sensor in a photonic crystal (PhC) slab. The device is composed of four cavities with a center wavelength at 3.3μm, and the channels are spaced by 10nm. The sensor is designed on an 800-nm-thick silicon-on-insulator platform for standard CMOS technology. The optical characteristics of the nanocavity structure are simulated by 3-D finite difference time-domain (FDTD) method. The transmittance of each channel is about 39%, with non-uniformity of transmittance across channels less than 0.25dB. The capability of the sensor is demonstrated by detecting carbon tetrachloride and benzene solutions, and shows a sensitivity of 209.2nm/RIU.