Traditional material balance methods for calculating coal-bed methane reserves did not consider whether the coal seam was saturated or not; actually, they were only applicable for the saturated ones. Therefore, a material balance method applicable for unsaturated coal seams has been developed in this paper. In the derivation, the initial gas content is expressed as a Langmuir's equation with respect to the critical desorption pressure, rather than the initial reservoir pressure. Finally, a linear material balance equation of the pseudo average reservoir pressure vs. cumulative produced gas is obtained, whose intercept on horizontal axis yields the initial gas reserves, and intercept on vertical axis gives the pseudo critical desorption pressure, instead of the pseudo initial reservoir pressure. By using this method to evaluate a simulated reservoir, its reserves can be obtained with an error less than 0.35%, which indicates the method is reliable when the average reservoir pressure, initial cleat porosity, Langmuir's volume and Langmuir's pressure are accurate.