As a new type of energy structure, marine power generation has become the main direction of future energy. Wave-energy magnetohydrodynamic power generation technology is widely used in the development of marine energy. To study the factors influencing the zero-load voltage of a novel piston-type magnetohydrodynamic (MHD) ocean power generation device, a three-dimensional flow model was developed for the power generation device. Because a Hartmann layer existed in the rectangular power generation channel, the internal structure of the power generation channel was improved by changing the traditional rectangular power generation channel to a piston-pipe channel. According to Faraday’s law and the hydrodynamic theory, the MHD module in CFD software was used conduct finite-element simulations of the generation device. The influences of the inlet velocity, the velocity distribution, and the current density on the zero-load voltage were investigated. The results showed that for regular wave motion, the zero-load voltage was positively proportional to the inlet velocity and the channel width. After the channel was improved, the optimal zero-load voltage and the minimum relative error were obtained when the inlet velocity was 3 m/s and the channel width was 60 mm.