A compact ammonia gas sensor with two gas chambers is proposed in this paper, whose core sensing device is a U-bending microring resonator. The waveguides of ring part and feedback part in this resonator are made of silicon on insulator (SOI) ridge waveguide covered with ZnO nanocrystals which are sensitive to ammonia gas. The sensor can measure two groups of gas samples simultaneously. By computer simulation, we obtain the clear sensitivity curves of two gas chambers in ammonia sensor when the gas concentration increases from 0 to 4‰. The gas concentrations in two chambers can be obtained from one output spectrum, which significantly reduces the material and time consumption.