Abstract: Coalbed methane(CBM)wells must drain water from coalbeds before producing gas. The gas production rate increases suddenly rather than slowly at breakthrough of gas at wells,which is a strange phenomenon so far people do not know its mechanism. In order to understand production characteristics of CBM,the mechanism of production rate outburst of CBM wells was analyzed. It is recognized that coal rocks are dual porosity media in microscope,consisting of cleats(fractures)and matrix blocks. Cleats and matrix pores of coals are saturated with formation water. CBM exists in the matrix with the form of adsorption,which can be exploited only after desorption from matrix under pressure drawdown through draining formation water. A little of desorbed gas cannot flow due to obstacle of capillary pressure,which are dispersed in water with a small saturation. As desorbed gas is increased and accumulated into continuous phase with a high saturation,the gas pressure and the flowing ability are also increased simultaneously. Coal matrix pores are quite small with high capillary pressure,which can trap massive desorbed gas in coal matrix. When increasing gas pressure breaks the capillary pressure,the massive desorbed gas flows suddenly into cleats of coal rocks,which results in the outburst of gas production rate in CBM wells. The production pressure of CBM is lower than its desorption pressure. The desorption pressure of CBM is actually the saturation or bubble pressure of formation water. Abrupt increase of production rate may damage coalbed or production tubes,so it is necessary to take a possible control of production rate in practice.