The neck tube is a critical load-bearing component in vertical cryogenic insulated cylinders, serving as the primary structural connection between the inner and outer vessels. Due to geometric discontinuities and extreme temperature gradients, the neck tube region is highly susceptible to thermal stress concentration and structural failure. This study presents a detailed thermal–structural finite element analysis of neck tubes in vertical cryogenic cylinders using ANSYS software. The effects of combined loading conditions—including internal pressure, self-weight, temperature difference, and lateral inertia load—are investigated for various neck tube geometries and height-to-diameter (H/D) ratios. Parametric studies are conducted to evaluate the influence of neck tube diameter, length, and reinforcement ring dimensions on the maximum Von Mises stress. The stress distribution characteristics and critical failure locations are identified, with particular emphasis on the reinforcement ring’s role in reducing stress concentration. The results provide optimized design guidelines for improving the structural safety and reliability of cryogenic insulated cylinders operating under severe thermal and mechanical conditions.