The invention discloses a wall building robot with a telescopic arm. The wall building robot comprises a rotating base, a robot body, a vertical moving module, a platform plate, the telescopic arm, atelescopic power assembly, a clamp assembly and a mortar cup assembly. The vertical moving module, the platform plate, the telescopic arm, the telescopic power assembly, the clamp assembly and the mortar cup assembly are mounted on the robot body, the platform plate is installed on the vertical moving module, and a sliding rail and a rail sliding block arranged along the sliding rail are arrangedon the platform plate; the telescopic arm is fixed above the rail sliding block, the telescopic power assembly is installed in the telescopic arm, and the telescopic power assembly controls the telescopic arm to horizontally move along the sliding rail; and the clamp assembly and the mortar cup assembly are installed at the two side ends of the telescopic arm respectively, the clamp assembly comprises a clamp and a first motor set, and the mortar cup assembly comprises a mortar cup and a second motor set. Through the arrangement of the telescopic arm, the use is more flexible, bricks and mortar can be clamped at the same time, bricklaying can be conducted synchronously after plastering, the bricklaying procedure is simplified, the overall bricklaying efficiency is improved, and the mortaris prevented from being solidified in the bricklaying process.