To study the distortion effect of single-box twin-cell thin-walled box girders with cantilever plates， the distortion differential equation was established based on the plate-beam frame method， and the initial parametric solution of the equation under vertical eccentric concentrated load was derived. The concrete analytical formula of the distortion frame moment of inertia of single-box twin-cell box girder with cantilever plates was also presented. The analysis on the cantilever box girder and simply supported box girders examples was performed using analytical theory. The results show that the analytical solution is in good agreement with the results from existing literature and finite element analysis， which verifies the correctness of the analytical theory. Through the parameter analysis， it can be seen that the greater the thickness of the middle web， the stronger the ability of the twin-cell box girder to resist distortion. The transverse bending moment of the external webs is little influenced by the variation of thickness of the middle web， but the transverse bending moment of the middle web increases with the increase of its thickness. When the thickness of the middle web is equal to that of the external webs， the transverse bending moment of the middle web is approximately twice that of the external webs.