The effects of traditional on-site Post Weld Heat Treatment (PWHT) and Vibration Stress Relief (VSR) on reducing the residual tensile stress of the welding seam of a 100 t small crane pedestal flange cylinder are studied. The test results show that: the average stress relief rate of PWHT is 14.7%, and that of VSR is 37.32%-46.03%; the residual stress dispersity of VSR is smaller than that of PWHT and the stress homogenization effect of VSR is better. The VSR process is optimized, and the best process parameters adapted to the VSR of the flange cylinder are obtained. The direct and indirect economic effects of VSR are compared with those of PWHT. VSR equipment is simple, easy to move, easy to operate and suitable for on-site processing of large offshore structure, which means VSR is of good application prospect.