<p>Hydraulic technology has the outstanding advantages of easy pressure compensation and high power density. It is an indispensable part of subsea equipment, such as deep-sea operations and submersible propulsion. There are few studies on electrohydraulic servo valves (EHSVs) in the deep sea. In this work, a novel electro-hydraulic servo rotary valve is designed, and its mathematical model is established. The analysis considers the variations in physical parameters such as temperature, ambient pressure, and oil viscosity resulting from changes in sea depth. This study focuses on the deformation of the rotary valve and the consequent alterations in leakage and friction torque. The findings indicate that at a depth of 12000 m, the fit clearance between the valve spool and the valve sleeve is 0.00413 mm, representing a 17% reduction compared with the clearance in a land environment. Then, the response of the rotary valve to depth is analyzed. The results indicate that the bandwidth of the rotary valve decreases with increasing depth. This study provides a reference for the use of the EHSV in the deep sea.</p>