The subsea production system is the preferred solution for the development of deepwater oil and gas fields, and the umbilical is the key equipment for subsea production system. The development of ultra-deep water and long-distance oil and gas fields has brought about the demand for high-voltage composite and lightweight of umbilical. The cross-section design of the umbilical is the most critical part of the overall umbilical system design, which determines important parameters such as the weight of the umbilical and key mechanical properties. This paper analyzes the factors that need to be considered in the design of the umbilical cable section. Taking the development of the umbilical in an oil and gas field in the South China Sea as an example, the cross-section design of the lightweight and electric composite umbilical is carried out. The method of designing the umbilical cable section of the underwater production system in this paper can provide better design reference for other deepwater oil and gas field development and has good engineering application value.