The utility model relates to the technical field of tools and instruments, and particularly relates to a special tool for maintenance operation such as line releasing and tightening in power transmission construction and adjustment of guide line and earth line sags, i.e., an adjustable line clamping device for power transmission line releasing and tightening. A handle of the adjustable line clamping device is an annular handle made of a steel bar, end parts of the handle are fixedly connected with a front end of an upper arm through a bolt, and a rear end of the upper arm is fixedly connected with a rear end of the upper arm through a bolt, a front end of a front arm is connected with a rear end of a line clamping groove through a bolt, a front end of the line clamping groove is connected with a side arm through an adjusting bolt and is connected with an arc groove in the side arm, and the other end of the side arm is fixedly connected with the front end of the upper arm and the handle through a bolt to form an irregular quadrangle. Operation such as installation and demounting of the adjustable line clamping device can be completed without requiring personnel to go in and out of guide and earth lines. The adjustable line clamping device is simple in structure, safe, reliable, light and flexible, and is suitable for guide and earth lines of various types; safety performance and mechanical performance of the line clamping device are ensured; and labour intensity of operating personnel is greatly reduced.