The invention discloses an ultralow temperature heat energy efficient-recovery system of a sulfur-phosphorus integrated unit and aims at solving the problem that the heat energy of thickened sulfuric acid generated by an absorption tower of a sulfuric acid device in an existing sulfuric acid-producing phosphoric acid plant is difficult to utilize. The ultralow temperature heat energy efficient-recovery system comprises a phosphoric acid thickening heater and a sulfuric acid ultralow-temperature cooler; the cooling water outlet of the ultralow-temperature cooler is connected with the inlet of a negative-pressure flashing evaporator; the steam outlet of the negative-pressure flashing evaporator is connected with the steam inlet of the phosphoric acid thickening heater; the condensate outlet of the phosphoric acid thickening heater is connected with the inlet of a closed water tank; and the outlet of the closed water tank is connected with the cooling water inlet of the ultralow-temperature cooler by use of a hot water pump. The ultralow temperature heat energy efficient-recovery system of the sulfur-phosphorus integrated unit is simple in structure, simple and convenient in system control and high in reliability; as a result, the heat energy of hot sulfuric acid can be well utilized, so that the steam consumption of the phosphoric acid device can be reduced or no steam can be consumed, and the purposes of energy conservation and consumption reduction can be achieved.