This study evaluates the power harnessing performance of a two-directional inertial pendulum energy harvester for miniature underwater mooring platforms (UMPs). This novel harvester can achieve a dominant rotational motion under different parametrical excitations. The two-directional energy harvesting performance is investigated through vibration platform experiments over roll-pitch coupling excitations. The frequency ratio between the pitch and roll motion is about 2 to 1, the roll excitation frequency is over the range of 0.2–0.3 Hz, and the pitch amplitude is much lower than the roll amplitude. Experimental results indicate that the inclination configuration of the harvester can improve two-directional energy harvesting performance significantly with low-frequency vibration. Geometrical tunability through adjusting inclination configuration for the harvester can enhance the two-directional energy harvesting apparently. Four prominent phenomena of the two-directional energy harvesting performance are observed with the increasing load resistance. The motion state transformation of the inertial pendulum can result in dramatical jumps or drops in two-directional energy harvesting.