Underwater direct-drive wave energy converter (UDDWEC) using linear-rotating axial flux permanent magnet generator (LR-AFPMG) can change form of energy conversion and improve conversion efficiency. This paper proposed a maximum power point tracking (MPPT) control based on inertia force in electromagnetic force for UDDWEC which can control the form and magnitude of electromagnetic force including damping force, spring force and inertia force. By adding inertia force in electromagnetic force, the pole location and dynamic response performance can be changed and adjusted. This paper studied the influence of inertial force in electromagnetic force and system response in detail. The research also involved the influence of buoys with different shapes on wave energy capture, the expression of q-axial reference current when inertia force is considered, the necessity of additional damping in the system and power capture width in different sea conditions. According to the hydrodynamic parameters and wave characteristics, the proposed MPPT control was verified through simulation in regular wave and JONSWAP spectrum and the results showed that MPPT control based on inertia force can capture the maximum wave energy and achieve maximum power capture width. Above on this, this control strategy can be a decent candidate for UDDWEC. Finally, a series of flume experiments were carried out to verify the accuracy of hydrodynamic parameters of underwater buoy.