This study addresses the limitations of buoyancy factor and compensation capacity in pressure hulls for full-ocean-depth underwater gliders operating in extreme deep-sea conditions. A novel lightweight multifunctional composite structure pressure hull (CSPH) is proposed, utilizing a carbon fiber cylindrical shell as the primary load-bearing structure and silicone oil as the buoyancy compensation medium. A mechanical model of the carbon fiber cylindrical shell under hydrostatic pressure was developed based on three-dimensional elastic mechanics theory. Furthermore, a comprehensive performance evaluation model for the CSPH was created, incorporating both the buoyancy factor (B_f) and buoyancy fluctuation coefficient (f_B). The NSGA-II optimization algorithm was employed to simultaneously minimize B_f and f_B by co-optimizing the carbon fiber ply parameters and the silicone oil volume (V_C). This optimization resulted in a Pareto optimal solution balancing buoyancy and compensation performance. The accuracy of the mechanical model and optimization results was validated through finite element analysis and pressure testing. The results show that, compared to traditional metallic pressure hull designs, the CSPH reduces the buoyancy factor by 48% and enhances buoyancy compensation performance by 2.5 times. The developed CSPH has been successfully deployed on the “Sea-Wing 11000” full-ocean-depth underwater glider, significantly improving its endurance and motion stability for long-term deep-sea observation missions.