Field-tracing based numerical simulation technique is investigated to design and analyze ultra-small self-focusing optical fiber probe. Firstly, the concept and principle of the field-tracing are described. Secondly, the method is discussed to implement the field-tracing technique in the physical optical software of VirtualLab. Finally, an ultra-small self-focusing optical fiber probe is simulated in the field-tracing based optical software of VirtualLab. In this paper, we find that under the conditions of a fiber spacer length of 0.36 mm and the self-focusing fiber lens lengths of 0.1 mm, 0.11 mm and 0.12 mm, the working distances of the probe are 0.75 mm, 0.63 mm and 0.51 mm, and the focus spot sizes are 32 μm, 24 μm and 19 μm respectively. The simulation results are in good agreement with the experimental data, showing that the field-tracing based numerical simulation technique is an effective tool for investigating ultra-small self-focusing optical fiber probe.