Abstract: The equivalent load resolution method was used in the study. The mechanism of shear lag effect of long-span continuous rigid-frame induced by vertical loads are discussed. The shear flow equilibrium equations of the box-beam are derived by use of three-rod analogous procedure. The shear-lag coefficients of the main controlling cross-sections of the continuous rigid-frame of Luzhou Second Bridge over the Yangzi River are obtained based on static loads measured on site.