The deep sea oil-filled brushless DC (BLDC) motor operates in an underwater environment. The pressure varies with the working depth. The viscosity of the oil in the motor is sensitive to the pressure and hence the viscous drag loss varies with the pressure. The high pressure has influence on the loss characteristics of silicon steel and results in the increase of the core loss in the motor. Computational fluid dynamics (CFD) method is adopted to calculate the flow field in the air gap and the viscous drag loss at different pressures is obtained. Finite element method (FEM) is adopted to obtain the core loss at high pressure. The motor design considering the high pressure effect is compared with the motor design neglecting the pressure effect. The comparison results show that high pressure has large influence on viscous drag loss and core loss, resulting in the decrease in the efficiency and increase in the temperature rise. The experiment is done and the results verify the effectiveness of the calculated results.