A dehydrogenation method of CO2 charge gas for synthesizing urea includes the following steps: firstly, using active carbon or iron oxide normal-temp. desulfurizer to remover H2S from CO2 charge gas, then using the normal-temp. hydrolyst to hydrolyze orgnaic sulfur at 40-100 deg.C, then using active carbon or iron oxide refined desulfurizer to make desulfurization up to total sulfur content in the gas be less than 0.5 ppm, then using dehydrogenation catalyst containing Pt-Pd dual noble metals or TH-3 type rare earth metal element-contained Pt-Pd dual noble metals dehydrogenation catalyst to mare dehydrogenation, removing H2 from CO2 charge gas and making H2 content be less than 300 ppm under the conditions of that its inlet temp. is 120-250 deg.C and pressure is less than 20 MPa. The said method is applicable to urea plant using coal or heavy oil to produce gas, can eliminate the hazard due to accumulation of H2, and can prolong catalyst life to about 5 years.