Mechanoluminescence (ML) sensors offer full-field strain/stress measurements and have the advantages of easy implementation, non-invasiveness, and low cost. However, knowledge of the mechanism of carrier detrapping owing to mechanical forces remains elusive as it may include one or more complicated processes. Several attempts to develop calibration models for converting the light emitted by ML sensors into mechanical stress have been reported, but few studies have tested the ML response under static or quasi-static loading rates. In this study, we built a real-time measurement system to evaluate the constitutive strain-luminescence relationship of ML sensors. The ML response was investigated at loading rates as low as 5 µm/s (i.e., strain rates as low as 16.2 μst/s). The results showed that the light of ML sensors was still excited under quasi-static loading rates. A double-exponential function was applied to describe nonlinear changes in light intensity over time. We used this function to modify the previous model and predict the ML responses under variable loading rate conditions. The results predicted by the model proposed in this paper are in good agreement with the experimental results.