Process of directly preparing urea by tripoly cyanamide

This invention relates to a urea preparation methods which directly use melamine assembly end gas, including high pressure synthesis, midst-low-pressure decomposition, condensing pressure, low pressure condensate recovery steps. First absorbing and condensing melamine assembly end gas (composition: NH3: 40 to 45%; CO2: 35 to 40%, and the rest of H2O) into ammonio liquid; and then mix ammonio liquid with liquid ammonia and passive air as raw materials of urea synthesis; at last to preheat this mixture to 180 ~ 195 deg C, and sent into tower of urea synthesis of high-pressure, generate urea solution.