Abstract: In order to study a stability lost control of the fuzzy ball drilling fluid to the coalbed methane well drilling,based on three coalbed methane U shaped horizont al wells in the south block of Qinshui Basin were taken as the study object,with ball layer agent,fuzzy generated agent,gas-core generated agent,membrane generated age nt and several active formulations added in the fresh water,the paper analyzed the fuzzy ball drilling fluid affected to the well wall stability in the coalbed methane well drilli ng. The site study results showed that the fuzzy ball drilling fluid could realize and control the drilling fluid density ranging from 0.96 ~ 1.06 g 1 cm3,the hopper viscosity wa s 30 ~ 50 s and the dynamic plastic ratio was0.4~0.9 Pa/( m Pa.s). With a good rheology,volume scalable,powerful lubrication resistance reduction and other advantages of the fuzzy ball drilling fluid,the stability of the well wall could be well kept. With the control of a leakage lost rate below 2 m3/ h,the reservoir pollution could be effectively reduced. During the application process of the fuzzy ball drilling fluid in the three groups of U shaped horizontal connected wells,the three open horizontal section length W as 803.67 m in average,the real drilling time was 388 h,and the machine drilling speed was 2.5 m/ h. The well construction period of the U shaped well was 37 days in ave rage and was 23 days reduced than the design period. Therefore,the drilling efficiency was highly improved.