Optical coherence tomography (OCT) is an attractive modality in biomedical imaging systems due to its non-invasive imaging character. Since the image quality of OCT may be limited by the decrease of transverse resolution away from the focus spot, working distance tunable probe can be a strategy to overcome such limitation and maintain high transverse resolution at different imaging depths. In this paper, a miniature, working distance-tunable in-fiber OCT probe is demonstrated. The influences of the graded index fiber (GIF) length as well as the air cavity length on the working distance and the transverse resolution are simulated and discussed. Experimental results prove that the working distance can be tuned freely from 337.31 μm to 22.28 μm, producing the transverse resolution from 13.86 μm to 3.6 μm, which are in good agreement with the simulated results. The application of the probe in an OCT system for imagining a standard USAF resolution target is investigated in detail. The best resolutions for the standard USAF resolution target imaging are 4.9 μm and 6.9 μm in horizontal and vertical direction, respectively.