An analytical method was investigated to design the gradient-index (GRIN) fiber probe based on characteristic parameters of a Gaussian beam propagating through the probe. First, a typical model of the GRIN fiber probe was presented, consisting of a single mode fiber, a no-core fiber (NCF), and a GRIN fiber lens. Second, a complex beam parameter matrix transformation method was adopted to derive the mathematical expressions of characteristic parameters, such as beam waist location, beam waist radius, and Rayleigh range. Then, MATLAB software was used to analyze the impact of the length of NCF and the length of the GRIN fiber lens on the characteristic parameters. Finally, performance comparison was performed between the calculation results and the experimental data published previously. The calculation results are in agreement with the experimental data and thus validate the presented analytical method for designing GRIN fiber probes. In addition, the characteristic parameters of a Gaussian beam going through the GRIN fiber probe will stay the same when the length of the GRIN fiber lens increases every 1/2 pitch length, which could be used to decrease the fabrication complication of GRIN fiber probes by increasing the length of GRIN fiber lens periodically.