The invention discloses a construction method for a cathode protection device for an underground pipeline system. The construction method comprises the following steps of: 1, drilling downwards from the ground surface (101) above the outer part of an underground pipeline (1); 2, mounting a hollow external protection pipe (2) in a hole which is drilled in the step 1 to ensure that the front end of the external protection pipe (2) contacts the outer wall of the underground pipeline (1); 3, lowering down a waterproof sleeve (5), ensuring that a feed connector (402) which is fixedly arranged at the front end of the waterproof sleeve (5) closely contacts the outer wall of the underground pipeline (1) without moving, and welding; 4, welding the feed connector (402) which is fixedly arranged at the front end of the waterproof sleeve (5) and a metal part of the underground pipeline (1) together; and 5, constructing a sacrificial anode (7). The construction method is comprehensive in technology, has an obvious effect and is convenient to construct and operate. The invention relates to systematic engineering, so that a great number of contents are involved. The construction method is high in technical maturity and has predictable extremely high economic values and social values.