This paper uses a thermo-mechanical dispersion coefficient model in a small-scale aquifer to examine the advection- dispersion heat transfer process. Analytical solutions of advection-dispersion are derived and proved their correctness with experiment. The assessment results show that the coefficient of the thermal-mechanical dispersion of 1×10-2 W/(m?°C) can be selected as the critical point where thermal transport is affected. The distribution of thermal mechanical dispersion coefficients can be divided into non-ignorable triangular domain and ignorable polygon domain. The calibration scope of longitudinal dispersion degree of natural flow field is made clear in different aquifers. However, the research result of small scale thermal dispersion show that, under condition that the particle diameter and the seepage velocity are both at the upper limit, the maximum of longitudinal thermal dispersion degree is at centimeter order of magnitude, which is significantly different from the research result of thermal dispersion degree under outdoors of large scale conditions. This proves the existence of scale effect of thermal dispersion, and thus shows the direction of further research.