Two kinds of combined-cycle propulsions were studied to meet the requirement of a hypersonic vehicle that cruises at Mach number 6. One consists of T/RJ/DMSJ (turbine engine, ramjet and dual mode scramjet), while the other consists of PCT/RJ/DMSJ (pre-cooled turbine engine, ramjet and dual mode scramjet). The ranges and the flying time of the hypersonic vehicle which uses T/RJ/DMSJ and PCT/RJ/DMSJ were obtained under thrust weight ratio of 0.8 and 1.0 with a given mission. The performances of two operating mode of PCT/RJ/DMSJ were compared. Results show that the range and the flying time of the two combined-cycle propulsions are almost close. The range of PCT/RJ/DMSJ is 3.6% higher than T/RJ/DMSJ and the flying time of PCT/RJ/DMSJ is 3.8% more than T/RJ/DMSJ under thrust weight ratio of 0.8. When the thrust weight ratio is 1.0, the range and the flying time of PCT/RJ/DMSJ are 4.6% and 4.8% higher than T/RJ/DMSJ, respectively. Under the lower thrust weight ratio, the fuel consumption of the transonic flying phase is the largest part during the acceleration and climbing process. The fuel consumption of the transonic flying phase decreases as the increase of thrust weight ratio and then more fuel can be used in the hypersonic cruise and the cruise distance increases. The range increases and the flying time decreases as the increase of thrust weight ratio.