In this paper, the uncertainty of convex lens focal length measured based on the Gauss formula method (real object-real image method) is analyzed comprehensively considering the factors of the depth of field, thickness, and spherical aberration of the lens, and a method to correct the systemic measurement deviation caused by lens thickness and spherical aberration is proposed. For the selected convex lenses with a nominal focal length of 25.0 mm, the measurement uncertainty decreases first and increases afterward with the increase of the object distance, and the minimum value before correction is 1.26 mm, where the spherical aberration of the lens has the most prominent influence on the measurement uncertainty. After the system measurement deviation being corrected, the change trend of the measurement uncertainty remains unchanged and the minimum value is reduced to 0.10 mm, which indicates that the deviation correction improves the accuracy of the measurement results greatly. The results of uncertainty analysis show that the measurement results are more reliable when the object point is placed near the twice focal length of the lens, providing a guide for experimental operation.