Abstract: To realize the lightweight design of partially encased steel-concrete composite beams (PEC beams), an innovative lightweight steel-lightweight aggregate concrete PEC beam (LW-PEC beam) was proposed in this study. The LW-PEC beam is formed by filling lightweight aggregate concrete into H-section steel with large width-to-thickness ratio. Six LW-PEC beam specimens were designed and four-point static loading flexural tests were conducted, considering the effects of width-to-thickness ratio and links spacing. Therefore, the test results were obtained, which include the failure modes, stress-strain characteristics, ultimate load capacity, and ductility of the specimens. The study investigated the influence of various structural parameters on their flexural performance. The results reveal that the failure mode of the LW-PEC beams is primarily characterized by concrete crushing and bruising in the compression zone of the pure bending segment, local buckling of the compressed flanges and web, and the bending and exposure of longitudinal reinforcement and links. The LW-PEC beams exhibit sufficient load-bearing capacity and deformation performance, while its load capacity will decrease with a higher section width-to-thickness ratio and denser links spacing, and the impact of links spacing on load capacity is ignorable. Ductility will decrease as the width-to-thickness ratio and links spacing increase, while the influence of links spacing on ductility will weaken as the width-to-thickness ratio increases. To investigate the composite action of the components in the LW-PEC beams, the load-bearing capacity was analyzed based on the strain distribution of each component. The ensuing calculations demonstrated a high degree of congruence with the experimental data. The analysis reveals that the components of the LW-PEC beams exhibit effective composite action. The H-section steel contributes most significantly to load-bearing capacity, in contrast, the contribution of concrete is relatively minor, its primary benefit is to increase overall load-bearing capacity by preventing local buckling of the H-section steel.