Abstract: To address the buckling issue in the web and lower flange near the first opening at the ends of castellated composite beams under fire-induced high temperatures, two methods are proposed: removing the end openings and adding stiffeners. Two specimens—one without end openings but with stiffeners, the other with regular openings—were tested under constant load during heating. Finite element models in ABAQUS were developed to simulate the temperature distribution, displacement development, and failure modes throughout the fire process. Practical recommendations were provided based on experimental and numerical results. Findings show: (1) Inclined (“八”-shaped) cracks form at the beam ends during early heating, followed by transverse cracks. During initial cooling, reverse-inclined cracks and three axial-parallel cracks appear; (2) Removing end openings reduces mid-span displacement by 43.10% under high temperature compared with beams with regular openings, effectively mitigating local buckling. (3) Web stiffeners reduce the mid-span displacement by 24.70% during fire exposure. (4) Thermally induced axial force under rigid restraint, combined with vertical load, increases deflection. (5) Simulations indicate that greater load, longer span, and thinner concrete slab decrease the fire resistance. Stiffener thickness exhibits a threshold effect: improvement beyond 9 mm is marginal. Stiffener placement near beam ends improves the fire resistance more effectively than uniform distribution.