The invention provides a method and a device for implementing primary rainwater closure in a municipal network system. In the method, primary closure rainwater at each rainwater port in a water catchment area is taken as an object, and a floating ball control chamber and a main flow-splitting chamber are arranged on two sides of the rainwater port. When a rainfall runoff is formed, heavily-polluted primary surface chemical runoff is subjected to quantitative closure to the municipal sewage pipe network by using the difference between the two chambers and a rainwater inspection well in vertical elevation. A floating ball control valve is arranged at a primary rainwater inlet of the main flow-splitting chamber; the floating ball water level in the floating ball control chamber controls the opening and closing of the floating ball control valve; the flow-splitting effluent flows into the municipal sewage inspection well in the neighborhood; and the split flow can be realized by the effective volume of the floating ball control chamber and the opening and closing of the floating ball water level control valve. By integrally arranging the primary rainwater closure and the rainwater port, inspection well and other closure devices in the municipal network system, the method and the device can effectively realize the closure control of the primary polluted rainwater under various working conditions of actual rainfall.