Fall dormancy plays important roles in the evaluation of alfalfa’s winter hardiness and in the selection of alfalfa breeding. A rapid and effective method to estimate the fall dormancy rating of alfalfa will shorten the breeding cycle. The purpose of this study is to test the correlations between the germination thermal time model parameters and the fall dormancy ratings and to evaluate the potential of the thermal-based fall dormancy methodology. Alfalfa cultivars with a series of fall dormancy ratings were used to study the responses of seed germination at six constant temperatures (5, 10, 15, 20, 25, 30 °C). The results showed that all cultivars had a relatively high germination percentage at all temperatures and the optimal temperature is 25 or 30 °C. Germination rate and base temperature significantly increased with the fall dormancy rating of alfalfa cultivars while thermal time (θ_T) decreased with the fall dormancy rating. The extremely significant linear regression relationships between the germination rate, base temperature (T_b), θ_T, and fall dormancy rating indicated that it is convenient and straightforward to predict the fall dormancy rating of unknown cultivars or lines using thermal time model parameters. This method can significantly shorten the selection and breeding cycles in alfalfa cultivation.