The invention discloses an LNG compressor under-pressure starting determining method and relates to the field of compressor design. The LNG compressor under-pressure starting determining method mainlyaims to increase the accuracy rate of LNG compressor set under-pressure starting determining. The method comprises the steps that according to starting calculation input parameters of an LNG compressor set under the under-pressure state, inlet flows of a compressor at all moments are subjected to iterative calculation; when the inlet flows at the moments converge, and according to the converged inlet flows, the inlet pressure of the compressor set at the moments is subjected to iterative calculation; when the inlet pressure of all moments converge, the rotating speeds of the compressor set atthe moments are subjected to iterative calculation; if the rotating speeds at the moments meet the rotating speed increasing requirement, the system performance parameters of the compressor set at the moments in the rotating speed increasing process are subjected to iterative calculation; and if the system performance parameters accord with preset system performance conditions, it is determined that the LNG compressor set can be started under the under-pressure state. The LNG compressor under-pressure starting determining method is suitable for LNG compressor set under-pressure starting determining.