The invention relates to a method for deducing the flow velocity distribution under an ice sheet. The method for deducing the flow velocity distribution under the ice sheet comprises the following steps: selecting a measurement section of a typical river channel; selecting time points for recording the measurement section; recording a measurement section status; obtaining flow velocity distribution data; calibrating parameters: taking the flow velocity of each measurement period into the formula; deducing the flow velocity and flow distribution; and accumulating data constantly. According to the method for deducing the flow velocity distribution under the ice sheet, a comprehensive measurement is performed by combining climatic conditions, ice sheet characteristics, and ice freezing stateon a typical river channel, and a number of parameters required for calculations related to the conditions are calibrated through a flow velocity expression. In the freezing period of each subsequentyear, only the climatic conditions, the ice sheet characteristics and the ice freezing state need to be compared to find the corresponding calculation parameters, so that the same flow velocity expression can be used for obtaining the velocity distribution under the ice sheet, and the digging of the ice surface is not required; therefore, the situation that the flow velocity is not consistent withactual flow conditions caused by artificially dividing the sub-ice flow velocity into the ice sheet area and the river bed area is solved.