The invention relates to the field of hydrology and hydroelectricity, and discloses a reservoir dynamic limited water level ascertaining method and system based on subtropic high intermediate-term movements. The method and system aim to manage the reservoir dynamic limited water level longer than prediction. The method comprises that at evolution periods of at least one associated region, longitude ranges of western ends of the ridges of the subtropic high corresponding to the evolution periods in the east-west direction and the varying function of the longitude of the western ends of the ridges of the subtropic high depending on time are determined according to historical change lows of the subtropic high intermediate-term; longitudes of western ends of the ridges of the subtropic high when precipitation occurs in a reservoir are as division points, so that the evolution period of arbitrary reservoirs affected by the subtropic high in the associated region is divided into six time periods, and the average daily precipitation of the reservoir corresponding to each time period is calculated; the limited water level of the reservoir is planed based on the six time periods according to the varying function of the longitude of the western ends of the ridges of the subtropic high intermediate-term depending on time and the average daily precipitation of the reservoir corresponding to each time period.