Abstract: For analyzing the shear lag effect of continuous box girder bridge with variable cross section, a 4 degree-of-freedom segment element of box girder beam was proposed. The stiffness matrix and equivalent node force array of the element were derived according to the homogeneous solution of the governing differential equation with additional deflection, and a corresponding solving program was developed with FORTRAN. The program was used to calculate examples of the simply supported box girder available in the literature. The shear lag effect of a three-span continuous box girder bridge with variable cross section was computed by the present program under lane load acting on the main span. It is shown that the shear lag coefficients in the middle section of the main span and the section of middle support reach 1.176 and 1.148, respectively. It is also shown that the positive and negative shear lag transition phenomenon occurs at the inverted bending point.