<p>Experimental evidence suggests that high temperatures affect cognitive performance; however, there are limited quantitative models available to evaluate or predict this effect. In previous studies, the authors proposed a thermal stress comprehensive index (TSCI), which reflects the coupling of physiological and psychological parameters. Based on this, a comprehensive model of thermal stress and cognitive performance (TSCI-CP model) under high-temperature conditions was established, and its rationality for moderate activity intensity tasks was verified. The present study further investigated the applicability of the TSCI-CP model for low activity intensity tasks. By integrating experimental data from a series of climate chamber experiments conducted at five temperatures (26, 30, 33, 37, and 39 °C) and two humidity levels (50% and 70%), the relationship between physiological and psychological parameters and the cognitive performance of 104 subjects at a low-activity level (sitting) was analyzed. Subsequently, the TSCI-CP model for low activity intensity tasks was established. The results of the goodness of fit and deviation analysis confirmed the validity of the model. Based on the TSCI-CP model, a new perspective on the relationship between thermal stress and cognitive performance under high-temperature conditions was proposed: the speed of cognitive tests exhibited a U-shaped trend, while the accuracy followed an inverted U-shaped trend. This study offers a theoretical foundation for ensuring the safety and security of low activity workers in high-temperature environments.</p>