We numerically analyze the performance of a liquid metal MHD power generation system for various external piston forces (regular: constant, sinusoidal, square; irregular: Piersion-Moskowitz) with three different working fluids (mercury, U-alloy47, and NaK78). Considering the shape loss due to different cross section areas between the MHD generator and piston driver parts, NaK78 prevails in performance (power and efficiency) for all kinds of forces owing to its relatively low density and high electrical conductivity. For regular and irregular periodic forces, when a piston period becomes long (comparing with the rise time), the power increases and the electrical efficiency approaches that for a constant piston force; For short periods, on the contrary, the electrical efficiency recovers that for no shape loss with limited effects of shape loss on the power. The irregular force input based on the P-M wave can give no big difference with the regular forces when the effective value of the input is the same. In summary, choosing a liquid metal with low density and high electrical conductivity to work under a relatively long period as well as curbing the shape loss as much as possible is needed to improve the power and the electrical efficiency.