The invention discloses a lineman climber with an adjustable opening size, which comprises a main clamp head, a bent pipe and a pedal which are sequentially connected, wherein the main clamp head is in sliding connection with the bent pipe through a connecting sleeve; the bent pipe is fixedly connected with the pedal; an auxiliary clamp head is movably connected with the pedal in the direction ofthe main clamp head; an electric adjusting mechanism is detachably arranged on the connecting sleeve; the electric adjusting mechanism comprises a box body; a power mechanism is arranged in the box body; a power output end of the power mechanism is connected with and controls a driving mechanism; the driving mechanism is meshed with a rack fixedly connected with the main clamp head; an extension button and a contraction button are arranged on the box body and connected with a power supply circuit of the power mechanism to control the forward and reverse rotations of the power mechanism. According to the invention, the opening degree of the climber can be conveniently and quickly adjusted. The lineman climber is suitable for the technical field of power line construction and installation.