Considering the narrow space, the design difficulty of the medical endoscope objective lies in performing local optical zoom and distinguishing the imaging view of locally enlarged human tissues. Therefore, the objective lens must meet the requirements of low chromatic aberration, high reliability, and miniaturization. From the analysis of the technical route for an endoscopic optical zoom, this study proposes the application of the Alvarez lens in the endoscope to achieve optical zoom. The proposed structure has the advantages of achieving simple zoom, stable structure, fast, and reliable imaging. First, we have developed the achromatic zoom optical design theory of the endoscope objective using the Alvarez lens based on the Gaussian bracket method and aberration theory. Then, we derived the initial optical design value of the achromatic objective lens system using the aberration theory. Finally, the optical chromatic aberration theory is used to verify the correctness of the theory, which provides an optical design theoretical basis for the further application of the Alvarez lens in the design of an endoscope.