Heat strain can reduce work efficiency and lead to heat-related diseases. In order to quantify the heat strain level of the sanitation worker in high temperature environments, a physiological state safety evaluation index (PSEI) was established. Firstly, the cusp catastrophe theory was introduced to analyze the changing trend of human physiological state under different thermal loads. Secondly, the information entropy method and variable weight theory were adopted to calculate the weights of the physiological indexes, and the cusp catastrophe progression method was used to establish human physiological state evaluation model. Finally, a new index-PSEI was proposed to quantify the safety level of the physiological state, and the safety interval of the PSEI was categorized. The results show that the weight of each physiological index varies with environmental conditions and physiological states. The weight range of mean skin temperature, tympanic membrane, systolic blood pressure and heart rate is 0.09 ~ 0.64, 0.08 ~ 0.41, 0.07 ~ 0.74 and 0.06 ~0.33, respectively. The safety level of PSEI is as follows: very safe (0.948, 1], relatively safe (0.886, 0.948], moderate (0.798, 0.886], relatively dangerous (0.649, 0.798] and very dangerous [0, 0.649]. The PSEI can be used to provide real-time physiological state evaluation and health warning for sanitation workers in high temperature environments.