Abstract: Because of the specificity of coal bed methane reservoir,it was easy to cause reservoir damage during drilling,it was important and practical significance to study the reservoir damage mechanism of coal bed methane reservoir.Based on the analysis of characteristics of coal bed methane reservoir in Qinshui Basin of Shanxi Province.The main reservoir damage mechanism during coal bed methane drilling was overbalanced drilling,capillarity phenomena,the invasion of filtrate and solids in drilling fluid,water sensitivity damage, polymer retention and incompatibility or poor compatibility between drilling fluid filtrate and coal bed water.The invasion velocity,the invasion radius and the invasion depth of filtrate and solids in drilling fluid with the increase of pressure difference during overbalanced drilling.The effective measures of reducing the degree of reservoir damage was to adopt underbalanced drilling technology,reduce the invasion of filtrate and solids in drilling fluid and prepare drilling fluid with the treated produced water and degradable polymer.