Adopting in-seam boreholes for gas pre-drainage is an important method for ensuring safety coal mine operation. However, borehole collapse often occurs during the process of construction in soft coal seams, especially in downward boreholes. The main purpose of this study is to develop an efficient drilling fluid for coal seam methane drainage boreholes. First, a drilling fluid with a formula of 1 wt.% xanthan gum, and 1.2 wt.% Na-CMC, 0.6 wt.% cellulase and 4 wt.% bentonite, which is simply referred to as the drilling fluid, was developed based on the actual requirements of the construction site. Then, five property tests (viscosity, fluid loss resistance, cuttings carrying capacity, anti-collapse performance and degradability) of the drilling fluid were conducted in the laboratory. Finally, engineering application performances were carried out in Tangshan Mine, China. The results indicate the following: (1) When the xanthan gum concentration is fixed, the viscosity of the drilling fluid improves with increasing Na-CMC concentration. The API filtrate loss of the drilling fluid decreases with increasing bentonite concentration, while the filter cake thickness increases and then stabilizes. (2) The drilling fluid has an efficient performance in the carrying capacity of drill cuttings and the anti-collapse performance of a coal mass. Thorough degradation is completed 300 min after adding cellulase. (3) Compared with traditional clear water, the effective drilling distance of downward core drilling using the drilling fluid increases by two times, and the anti-collapse performance for coal seam methane drainage boreholes is remarkable. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.