A three-dimensional numerical simulation about the separation flow in the high-altitude nozzle of a liquid rocket engine on ground was conducted. It obtaines the flow field parameters and the side-loads under work conditions from 8��MPa inlet total pressure to 1��MPa inlet total pressure. The results indicate that, with the reduction of inlet total pressure, the nozzle flow field will sequentially experience free shock separation mode and restricted shock separation mode. In restricted shock separation mode, the wall pressure has great fluctuation, and is even beyond the ambient pressure in reattachment point. In separated flow fields, side-loads will occur, the directions of side-loads are random. The calculation obtaines the biggest side-load under 4��MPa inlet total pressure. It can be predicted that the side-load peak will appear with the exchange between free shock separation mode and restricted shock separation mode.