This paper proposes a cavity adjustment method that corrects the relative misalignment of cavity parameters caused by the conversion of the initial cavity and test cavity structure during cavity ring-down high reflectance measurements. The proposed method monitors the shape of the ring-down cavity-transmission spot and also improves the efficiency of cavity adjustment. The cavity adjustment is based on the aspect ratio of the circumscribed rectangle and the area ratio of the spot to the circumscribed rectangle. The method suppresses the excitation of the high-order transverse mode caused by the misalignment of cavity parameters and maintains the transmission spot modes of the initial and test cavities in the TEM00 mode. In experimental tests, the adjustment based on the aspect ratio and area ratio not only distinguished different spot modes but also effectively dealt with special spot patterns between modes (such as elliptical spot and multimode jump). When the initial cavity and the test cavity spots were adjusted to the TEM00 mode, the measured mean reflectance of the sample was maximized and root mean square was minimized. Results show that this method can help to improve the cavity adjustment efficiency in cavity ring-down high-reflectance measurement technology.