Abstract: Spherical bulkheads are widely used in large-deep submersibles due to their high volume-weight ratio and material utilization. The sphere-toroid-cylinder combined spherical bulkhead has the advantages of a simple structural style, clear mechanical property and relatively low construction difficulty, resulting in great engineering applicability. In this paper, spherical bulkheads with sphere-toroid-cylinder combination are studied, and their stress strength characteristics under uniform external pressure are analyzed. The Riccati transfer matrix method is used for the calculation of spherical bulkhead structure by combining shell control equations. The paper is concerned with the stress strength analysis of the sphere-toroid-cylinder combined spherical bulkhead under external even pressure. To improve the performance of the structural strength of spherical bulkheads, the influences of parameters including sphere shell radius, toroid shell radius and boundary rigidity on the stress strength of the sphere-toroid-cylinder combined shell are discussed using the Riccati transfer matrix method. In order to depict the geometric figure and mechanical character of the bulkhead, the concept of 'flatness' is proposed, and the suggested range of 'flatness' is given. In this range, the mechanical constraints of the ring-stiffened cylinder acting on the sphere-toroid combined shell may be taken as simple support, thus simplifying the calculation of structural strength. The conclusions drawn from this paper can be used as reference points for the design of sphere-toroid-cylinder combined shell structures.