The invention discloses a fast mounting device for an HDPE water supply pipe and a connecting method thereof. The device comprises a first connection, a second connection, an outer connecting cap andan inner hexagon connection, wherein the first connection is used for sealing and fixing an HDPE water supply pipe, the second connection is used for sealing and fixing another HDPE water supply pipe,the first connection and the second connection are communicated through the outer connecting cap, the interiors of the first connection and second connection pipelines fix the HDPE water supply pipethrough the inner hexagon connection, two ends of the inner hexagon connection are respectively arranged on a first threaded ring and a second threaded ring, a first cavity body is arranged in the first connection, a second cavity body is arranged in the second connection, and the first cavity body and the second cavity body are used for the flowing of the water in the HDPE water supply pipe; according to the invention, the inner part of the HDPE water supply pipe is extruded outwards, and the outer part of the HDPE water supply pipe is extruded inwards, so that the pipe is locked doubly by aconnector, the impact of water pressure on the connecting position is shared together, the connector is convenient to carry due to a simple structure, and the threaded connection at each position is convenient to install.