Optical fiber gas sensing technology based on micro-ring resonator is a new measuring technique, which has a wide applicable prospect in fields of industrial production, environment monitoring and medical science. There is a micro-ring structure in the gas sensor based on absorption spectroscopy, which has some advantages of the whole compact fabric and perfect wavelength selectivity. According to numerical analysis about the characteristic of spectrum absorption of specific gas and micro-ring resonator, a normal design of optical fiber detection system of gas is presented and discussed, which has micro-ring optical filter. The theory of micro-ring resonator is mainly introduced. And the relations among the size of the micro-ring, resonance wavelength and freedom spectrum range are discussed in detail.