The invention discloses an 
orbit injection method of a two-stage-to-
orbit spacecraft with reusable first-stage energy, comprising the steps of 1, modeling the two-stage-to-
orbit spacecraft, wherein the first stage is a skylon using an SABRE engine as a 
power unit, and the second stage is a small 
rocket having effective 
payload that accounts for 5.89% of 
takeoff total 
mass; 2, carrying out a first-stage 
takeoff phase; 3, carrying out a first-stage suction mode phase that allows climbing to 25 km in suction mode; 4, carrying out a first-stage 
rocket mode phase, wherein the SABRE engine switchesto the 
rocket mode and propels the two-stage-to-orbit 
spacecraft to continuously 
climb to 100 km, separating the first stage and the second stage, and allowing the first stage to return to the groundfor use in an unpowered glide mode; 5, carrying a second-stage 
vertical climbing in the two-stage climbing phase, and delivering the effective 
payload to a low-earth orbit of 300 km at the speed of 7700 m / s. With the technical difficult and launching cost comprehensively considered, two-stage-to-orbit is achieved based on the SABRE engine, while the first-stage energy is reusable; the effective 
payload is significantly increased.