This paper deals with the numerical calculation of flow in the Hydraulic model tank of ballast water exchange by pumping-through method. It is demanded by the IMO that ballast tank water from the ships should be exchanged using Pumping -Through method until 2016. By this method, it is assumed that 95% of the ballast water exchange in the tank is possible if seawater of 3 times of ballast water discharge using this method. In our previous work, the hydraulic model experiment of ballast water exchange by pumping-through method was carried out. The ballast water tank was 2-dimensional profile. The experimental results showed that pumping-through method can dilute about 95% of ballast water if water volume of 3 times of the model tank was overflowed. The numerical calculation of flow in the hydraulics model tank of ballast water exchange by pumping-through method was carried out by the differential method. The calculated result was compared with the experimental result. The both results was very agreement.