Thermal discomfort has a great dynamic impact on cognitive performance and occupants’ well-being during prolonged working. However, few studies have focused on the dynamic changes in cognitive functions and physiological responses under extreme heat stress. This study investigated the cognitive impairment over exposure time at various ambient temperatures, ranging from 21.15℃ WBGT to 38.93℃ WBGT, and its underlying physiological mechanism. 30 well-trained volunteers and 8 occupational operators participated in the experiments, with typical cognitive tests, ECG, and fNIRS measurements. Negative correlations were mainly observed between cognitive test accuracy, ECG RRI, ECG RMSSD, ECG HF/LF, fNIRS ALFF, fNIRS functional connectivity (CORRz, COH, PLV), and exposure time. This indicated that increased physical load, stress, and decreased regulation ability to sudden changes, cognitive resource input, and inter-regional brain cooperation were key factors contributing to cognitive impairments over time. A comprehensive performance index (CPI) was established by the weighted sum of accuracy in cognitive tests and physiological features of ECG and fNIRS. The weights suggested fNIRS ALFF, ECG RRI, and fNIRS functional connectivity could be potential biomarkers for cognitive assessment. A set of linear quantitative CPI curves over exposure time was supplied to calculate the thermal tolerance exposure time at various risk tolerances and ambient temperatures. A power-law relationship ( ) was observed between the thermal tolerance exposure time and the standardized temperature. This study provides a physiological insight into cognitive impairment under prolonged heat exposure, and quantitative thermal tolerance exposure curves to evaluate safe thermal working duration for both low-risk groups (officers and drivers) and high-risk groups (operators in industry)