Abstract: When a horizontal well is drilling through the No.15 coal seam in the Qinshui Basin, coal seam collapse and instability and reservoir contamination tend to happen. To deal with these problems, degradable thickening agent and low-temperature oxidizing breaker were developed. Then, a set of sold-free drilling fuid system which can guarantee both borehole stability and reservoir protection was prepared. And fnally, this system was evaluated by performing experiments on its rheological property, fltration property, inhibition property, reservoir protection and gel breaking capacity. It is shown that this system is characterized by stable property, strong inhibition and good compatibility with the formation fuid. The recovery ratio of coal rock is higher than 96% and its relative expansion ratio is lower than 0.3%. The gel breaking ratio is 95% at 30 ℃, and the recovery ratio of coal rock permeability after gel breaking is over 96%.This drilling fuid system was applied on site in 4 U-shape horizontal wells in Qiyuan Coal Mine. The drilling operation was smooth and no accident happened. The skin factor of well testing after drilling was the lowest (-2.17). And thus, the double targets of borehole stability and reservoir protection are realized.