Background: The morbidity and mortality rates from heat illness have increased due to a higher number of heatwaves. Effective urgent care of heat illness is crucial for optimizing patient outcomes. However, few studies have examined the emergency preparedness measures required for treating such patients. Methods: From December 23, 2019, to January 23, 2020, a content-validated instrument containing the Perceived Emergency Preparedness Scale for heat illness (heatPEPS) was administered to emergency nurses in China through WeChat. Some of these nurses were retested two weeks later. SPSS 26, IRTPRO 4.2, and NVivo 12 Plus were used for data analysis. Results: In total, 46.4% (200/431) of the participants returned valid responses. With dichotomous scoring, a high score for heatPEPS (mean 7.29; SD 1.667) was elicited. The reduced 9-item heatPEPS had a perfect fit with the 2PL model (M-2 = 27.24, p > 0.05; RMSEA = 0.01) and acceptable internal (alpha = 0.68) and test-rest reliability (intraclass correlation = 0.56). Many participants (74%) were dissatisfied with their heat illness-related knowledge and skills, suggesting an area that could be improved for better emergency preparedness. Conclusion: Emergency departments appear to be well-prepared; however, this is subject to social desirability bias. The 9-item heatPEPS is a reliable and valid tool to measure emergency preparedness for heat illness.