The invention discloses a C-shaped electric resistance welding gun with a cooling water pipe embedded inside. The C-shaped electric resistance welding gun comprises a support, a fixed arm fixedly arranged on the support and a servo electric cylinder fixedly arranged on the support. A first electrode is fixed to a piston rod of the servo electric cylinder, and a second electrode is fixedly connected to the end of the fixed arm. Grooves are formed in the fixed arm, the cooling water pipe is embedded in the grooves and is a dual-layer pipe, the cooling water pipe comprises an inner pipe and an outer pipe, and cooling water flows in from the inner pipe and flows out from the gap between the inner pipe and the outer pipe. One end of the cooling water pipe is fixedly connected with the second electrode in a sealed manner, the inner pipe communicates with the outer pipe in a chamber of the second electrode, and the other end of the cooling water pipe is fixed to the fixed arm. The grooves areformed in the fixed arm, the cooling water pipe is embedded in the corresponding grooves, no cooling water pipe or fastener is exposed outside the fixed arm and a second motor, space is saved, and interference with a to-be-welded workpiece, a clamp and other surrounding components is avoided. Sealing performance is good.