A new catalytic system, AlEt3-H2O-H3PO4(C), was successfully used as a catalyst for synthesizing poly (1,4-dioxan-2-one) (PPDO) in bulk via the ring-opening polymerization (ROP) of 1,4-dioxan-2-one (PDO). The effects of the molar ratio of monomer to catalyst (MIC), the polymerization temperature (T), the reaction time (t) and the water content of monomer PDO (W) were studied systematically. AlEt3-H2O-H3PO4 has been proved to be a very effective catalyst for ROP of PDO. High molecular weights of PPDO can be obtained under following conditions: T = 80 degrees C, M/C = 1311-1573, t = 8-10 hours and W< 80ppm.