The underwater glider has difficulty accessing the complex and narrow hadal trench for observation, which is affected by its limited regulation capability of pitch angle (−45°~45°). In this study, a compact attitude regulating mechanism is proposed to extend the regulation range of pitch angle from −90°to 90° and to install it on the hadal-class underwater glider Petrel-X^PLUS. Subsequently, the dynamics model of Petrel-X^PLUS is established using dual quaternions to solve the “gimbal lock” problem caused by the increased pitch angle range. Within the extended pitch range, the motion modes of the glider are enriched into long-range, virtual mooring, and Lagrangian float modes for long-range, small-area, and current-following observation missions, respectively, and are analyzed using the established dynamics model. Moreover, a ballast method was used to modify the pitch angle range and initial equilibrium state of a constructed underwater glider. Finally, Petrel-X^PLUS achieved a pitch angle regulation range of −90°~90° in a water pool experiment and completed three consecutive profiles in a sea trial in the Challenger Deep, Mariana Trench, with all depths over 10,000 m, of which the maximum depth was 10,619 m. The proposed mechanism and methods can also be applied to other submersibles to facilitate ocean observations.