A high-precision test method based on infinity optical transformation system was investigated to detect the focusing performance of a small fiber lens. Consider the ultra-small gradient-index (GRIN) fiber probe as an example of the small fiber lens, its characters of focusing performances are analyzed. The infinity optical transformation system was employed to amplify and capture the exit beam from the probe in the non-contact mode. Then the curve fitting method is used to deal with the experimental data. The result shows that with the given small fiber lens the obtained working distance and waist diameter in the experiment system are 0.72 mm and 35 μm, respectively, while the calculated working distance and waist diameter based on the theory are 0.75 mm and 33 μm accordingly. The experimental results are in good agreement with the theoretical results. Therefore, the proposed test method based on infinity optical transformation system is applicable to detect the focusing performance of the small fiber lens.