Abstract: The instalility of wellbore and associated coal bed collapse is the mainly concerned problem in coal gas drilling.Based on the engineering data in situ,the numerical simulation is conducted to investigate the size effect of horizontal wellbore in coal bed by using the Realistic Failure Process Analysis(RFPA^2D) coed.Numerical results visually shows the initiating and propagating of the fractures,the transfer and release of the stress near the vicinity of wellbore and the induced collapse of coal.lt is pointed out that the size of wellbore have a obvious influence on the magnitude of well pressure using ofr maintaining the stability of the coal bed.For a bigger wellbore,the higher well pressure is necessary.