Abstract: Liu-109 well is a 3rd spud-in appraisal well located in Liugou Village, Jiuxi District, Yumen Oilfield. A stuck drill bit accident occurs in this well during the unbalanced drilling operation of 3rd spud-in slim hole. The accident treatment is limited by borehole size and processing tools. The acidification stuck-freeing scheme is prepared after the analysis of rock samples of the stuck drill layer and the acidification rate test of samples. The test indicates that mud acid can make bonding materials, stratum debris or falling pieces between the stuck drilling tool and stratum become cracked, crushed, and loosened under acidification reaction, so as to complete the stuck-freeing of stuck drilling tool. The mud acidification stuck-freeing process is applied to this well and the accident thus is solved smoothly. This application provides certain experience and reference for processing the stuck drill accident of deep slim-hole drilling.