Abstract: This paper presents a simple method to estimate and optimize the maximum launch capability of low Earth orbit (LEO) with different launch modes and trajectories, combined with trajectory optimization. The numerical calculation is carried out for the CZ-7 rocket, to analyze the specific effect and the characteristics of the optimal launch trajectory when launching with the cross-feed technology. The results show that the maximum launch capability of the LEO can be increased by 3 t (about 20%) and the maximum axial acceleration during the launch can be reduced by nearly 50% if for the CZ-7, the propellant cross-feed technology is applied in the launching process. With the help of the propellant cross-feed technology, the launch capability of such lift launch vehicle can be greatly improved.