The invention provides a device and a method for on-line diameter expansion of plastic pipes. The device consists of a cooling water jacket (1), a vacuum sizing sleeve (2) and a cooling vacuum box (3), wherein the vacuum sizing sleeve (2) is arranged at the front end inside the cooling vacuum box (3), and the cooling water jacket (1) is arranged at the front end outside the cooling vacuum box (3).The device of the invention is used in conjunction with pipeline production equipment, cools pipelines immediately after extrusion molding through the cooling water jacket, and then directly expandsdiameter through the cooling vacuum box and the vacuum sizing sleeve arranged inside the cooling vacuum box, thereby changing a two-time sizing method which needs reheating in the prior art and enabling the plastic pipes to change size and have the memory function of heat shrinkage.