In order to investigate miniaturized probes for optical coherence tomography (OCT) imaging, the optical characteristic parameters of the gradient-index (GRIN) fiber probe are analyzed. The model of GRIN fiber probe is overviewed that consists of a single-mode fiber, a coreless fiber and a GRIN fiber len. The optical characteristic parameters (working distance, and focal spot size) of the probe are defined and solved by the complex beam parameter matrix transformation method. A method is proposed to validate the characteristic parameters experimentally. The results show that, setting the length of the coreless-fiber 0.48 mm and the length of the GRIN fiber len 0.17 mm respectively, the theoretical working distance and focal spot size are 1.05 mm and 28.2 μm accordingly. Given the same conditions, the experimental measured working distance and focal spot size are 1.0 mm and 28 μm respectively. Therefore, the theoretical values are in good agreement with the measured data, which validate the proposed optical characteristic parameters of GRIN fiber probe.