The invention discloses a water age measurement method for a semi-closed water area. The method comprises the following steps that: firstly, establishing the hydrodynamic force mathematical model of awater area to be measured; through the hydrodynamic force mathematical model, calculating the water depth and flow velocity information of each water quality point of the water area to be measured; constructing a convective diffusion mathematical model, and setting the boundary condition of the convective diffusion mathematical model; according to the hydrodynamic force mathematical model, calculating to obtain the water level and flow velocity information of each position of the area to be measured and the boundary condition of the convective diffusion mathematical model; calculating the concentration values of a conservative substance p and a non-conservative substance r of each moment of each water quality point of the area to be measured through the convective diffusion mathematical model; according to the concentration values of a conservative substance p and a non-conservative substance r of each moment of each water quality point of the area to be measured, calculating the instantaneous water age of each corresponding moment of each water quality point of the area to be measured; and finally, according to the instantaneous water age of each corresponding moment of each water quality point of the area to be measured, calculating a time average water age in each time period of each water quality point of the water area to be measured. By use of the method, the water age of the semi-closed water area can be efficiently, conveniently and accurately calculated.