Abstract: In order to discover the application of the modern production decrease analysis to the coalbed methane well, based on the systematic statement on the development of the modern production decrease analysis method, according to the production data of the coalbed methane well group in Yanchuan South Block, the fou r modern production decrease analysis charts of the Blasingame, Agarwal-Gardner, NPI and Transient were applied to quantitatively evaluate the reservoir permeabiity, effective crack half length and drainage radius and to study the relationship between the drainage radius and reservoir permeability, the effective crack half length as W ell as the effective crack half length and the total hydraulic volume and added sand quantity of the fracturing hydraulic. The results showed that the reservoir permeabilit y of the well group was ( 0.008 ~ 0.064) x10- 15m2 and the heterogeneity was strong. The effective crack half length of the fracturing was 27 ~ 72 m and the fracturing to tal volume and the added sand quantity were in a positive correlation relationship. The drainage radius of a single well was 36 ~ 103 m and the average was 56 m as W ell as was in a positive correlation relationship to the reservoir permeability and the effective crack half length. When the effective crack half length was longer than 50 m and the permeability was over 0.06x 10- 15m2, the drainage radius would be over 100 m.