The invention discloses a LNG compressor unit start-with-pressure determining system, and relates to the filed of compressor design. The start-with-pressure determining rate for an LNG compressor unitcan be improved. The system comprises an input parameter determining device, an inlet flow iterative computation device, an inlet pressure iterative computation device, a rotating speed iterative computation device, a performance parameter iterative computation device and a start determining device. The inlet flow of the compressor at each moment can be obtained through iterative computation through the start computation input parameters of the LNG compressor unit in the pressure state. When the inlet flow is converged, the inlet pressure of the compressor at each moment is obtained through iterative computation through the converged inlet flow. When the inlet pressure is converged, the rotating speed of the compressor unit at each moment is obtained through iterative computation; if therotating speed meets rotating speed improving requirements, the system performance parameters of the compressor unit at each moment in the rotating speed improving process are obtained through iterative computation. If the system performance parameters meet preset conditions, it is determined that the LNG compressor unit can be started with pressure.