To investigate the mechanical performance of large plate girders used in boiler-supporting steel structures under eccentric loading, nine 1:5-scaled closed-section HK-type and open-section π-type specimens were designed and tested. The specimens had a span of 5960 mm, an overall length of 6220 mm, and a depth of 1760 mm. Concentric loading and eccentric loading with eccentricities of 30 mm, 60 mm, and 90 mm were considered. The results show that under eccentric loading, the mid-span displacement increased with increasing eccentricity for both HK-type and π-type specimens. Compared with π-type specimens, HK-type specimens exhibited smaller mid-span displacements and higher secant stiffness under the same eccentric loading conditions, indicating better deformation resistance. At a load level of 3000 kN, the secant stiffness of the HK-type specimen decreased from 192.31 kN/mm to 172.81 kN/mm as the eccentricity increased from 0 mm to 90 mm, while that of the π-type specimen decreased from 156.57 kN/mm to 142.18 kN/mm. The measured transverse brace stresses of π-type specimens were higher than those of HK-type specimens, indicating stronger interaction between the π-type girder and the lateral-restraint system under the applied eccentric-loading conditions. Under the adopted eccentric loading and lateral restraint conditions, the HK-type specimens exhibited smaller mid-span displacements, higher absolute secant stiffness, and lower transverse-brace stresses than the corresponding π-type specimens. This study provides experimental data for understanding the deformation behavior, torsional response, and stiffness degradation of large plate girders in boiler-supporting steel structures.