The paper presents a mathematical model, a numerical method and calculation results of hydrodynamic forces and moments, acting on the hull of submersible object (SO), moving in internal wave field. The mathematical model and the software permit to take into account three-dimensional geometry of unsteady flow, its time and space heterogeneity created by internal wave, liquid viscosity and actual geometry of the hull. The force calculation results are given for SO hull at performing manoeuvres close to pycnocline at variation of single-mode internal wave parameters. Analysis of the results has confirmed the adequacy of the offered mathematical model and has allowed to give quantitative and qualitative assessment of internal wave effect on SO hydrodynamics. The received results can be used for SO design and while creating trainers, simulating SO dynamic in natural environment.