Abstract: In the process of oil and gas field development, some problems occur during the drilling of horizontal wells, e.g. difficult cuttings carrying, severe backing pressure and low rate of penetration (ROP), which seriously impact production benefits. The jet mill bit can not only reduce the bottom hole pressure difference to realize the underbalanced ROP improvement, but also crush the cuttings to realize suspended cuttings transportation, so as to provide the technical support for the safe and efficient drilling of horizontal wells. The numerical simulation model for the flow field of jet mill bit was established on the basis of fluid dynamics calculation method. Then, the flow field characteristics of jet mill bit and the influences of the size of its external cuttings drainage channel were numerically simulated by means of the standard k-ε two-equation model. It is indicated that when there is no external cuttings drainage channel, the depressurization effect of the jet mill bit is the best and the low- and middle-pressure zones at the bottom hole present the trend of one decreasing and one increasing. In addition, the distribution of bottom hole pressure along the diameter direction is extremely uneven, and high pressure and low pressure are crisscrossed. When the external cuttings drainage channel is opened, the bulk volume flow ratio of the internal cuttings drainage hole is over 100%. As the width of single external cuttings drainage channel increases, the bulk volume flow ratio of the internal cuttings drainage hole increases. Besides, the phenomenon of drilling fluid backflow occurs in the external cuttings drainage channel with the backflow volume flow ratio basically below 4%. The research results provide the theoretical basis for the optimal design and application of jet mill bit.