Taking No. 3 coal in Qinshui Basin as research object, factors such as solid phase of drilling fluid, water sensitivity, stress sensitivity, scaling, capillarity and polymer plugging which maybe damage coal reservoir were researched during coalbed methane drilling. Sample results analyzed show that No. 3 coal reservoir has low porosity and low permeability, high content of clay mineral. Lab evaluation results show water sensitivity, scaling, solid phase invasion of drilling fluid and polymer plugging are factors which induce the coal reservoir damage. Drilling fluid which has suitable salinity, suitable density, anti-scaling, and removable drilling fluid agents can protect coal reservoir from damage during drilling for pinnate horizontal wells. [ABSTRACT FROM AUTHOR]