Abstract: Well Cha-302 is a key appraisal well located in a gentle slope belt structure in the south of the Songliao Basin. This well penetrated in the third interval the Yingcheng formation which is developed with tuff into which mud is easy to lose. During drilling, lost circulation was encountered several times at depths 3,686 m, 3,726 m and 3,955 m. When drilling into the deeper Huoshiling formation, higher formation pressure required that the upper section be strengthened to increase the pressure bearing capacity to an equivalent density of more than 1.38 g/cm³. Since the tuff formation drilled has low formation pressure, the mud is easy to lose into this formation, and when mud losses happen, the conventional lost circulation control methods are unable to effectively control the mud losses. To overcome these difficulties, a low density cementable lost circulation slurry was adopted. Based on the severity of mud losses in different spots, the total concentration of the bridging agents, the ratio of the different particles and the cement strength of the lost circulation slurries were accordingly adjusted. For the three main points of mud losses, the formation strengths were gradually increased by a certain increment. The results of the field operation show that during formation strengthening, the pressure decreased twice significantly and then increased slowly, and finally the pump pressure reached 16.2 MPa. After drilling out the cement plug, the equivalent circulation density at the bottom was 1.38 g/cm³, and the lower Huoshiling formation was successfully drilled. This technique of controlling lost circulation is first used in the Chaganhua block in the south of the Songliao Basin, it effectively strengthens the tuff formation and provides a new technical clue for controlling mud losses.