When a chimney reaches over 210 m in height, significant errors will occur if the even curvature method is used to calculate additional bending moments. A method is proposed for precise calculation of the additional bending moments of the chimney, which is based on the theory of conjugate beam, along with the chimney stiffness being reduced. By taking a chimney project as a case study, the method is used to calculate the additional bending moments and the lateral deflections under the lateral wind loads. In order to verify the calculation results by this method, the ANSYS software is used to make a comparison, which has proved the reliability of this method.