There are a large number of mudstone strata in the Wulonggou mining area of Qinghai Province, which are characterized by high total clay content, ease to expand after absorbing water, and many micro-cracks in the strata. In the process of drilling, problems such as hole shrinkage, collapse and insufficient core recovery are easy to occur, which seriously affects the evaluation of the scale and economic value of the ore body. Therefore, it is of great significance to study the drilling fluid suitable for this area. A water-based drilling fluid suitable for mudstone formation has been developed through the orthogonal experiment. The drilling fluid formula was: 5% bentonite + 2% soda ash (Na2CO3) + 0.05% highly viscous polyanionic cellulose (HV-PAC) + 0.4% low viscous sodium carboxymethyl cellulose (LV-CMC) + 2% sulfomethyl phenolic resin (SMP-1) + 2.5% non-fluorescence anti-sloughing lubricant (FT-342) + 0.4% MMH (MMH) + 1% white oil. The drilling fluid had water loss of 4mL, and the expansion rate of mudstone was reduced to 3.02% in the drilling fluid, whereas it is 20.26% in clean water. The mudstone structure remained intact after soaking in the drilling fluid for 24h. The drilling fluid has the advantages of low filtration, strong inhibition and simple preparation process, which can be well applied to drilling of mudstone strata in the Wulonggou area.