The invention discloses a system for producing ammonia gas by hydrolyzing a urea solution. The system comprises a hydrolyzer, a hydrolyzer thermometer T1, a hydrolyzer pressure transmitter P, a hydrolyzer liquidometer L, a urea solution regulating valve A1, a hydrophobic heat exchanger, a steam regulating valve A2, a hydrolyzing gas outlet regulating valve A3, a steam heat exchanger, a hydrolyzing gas outlet thermometer T2, a hydrolyzing gas outlet flowmeter F, a steam hydrophobic valve and a hydrolysis system control module, wherein the hydrolyzer is provided with seven ports respectively connected with the components above; the hydrolysis system control module is respectively connected with the hydrolyzer thermometer T1, the hydrolyzer pressure transmitter P, the hydrolyzer liquidometer L, the urea solution regulating valve A1, the steam regulating valve A2, the hydrolyzing gas outlet regulating valve A3, the hydrolyzing gas outlet thermometer T2 and the hydrolyzing gas outlet flowmeter F through cables. The invention also discloses a control method for producing ammonia gas by hydrolyzing the urea solution, which is implemented through six steps. The system and control method disclosed by the invention are simple and reliable, and easy to control. Steam is taken as a heat source of a hydrolysis system, so that the operation cost and the energy consumption are reduced.