Construction workers often expose to heat stress hazards as temperature and humidity increase. Heat-related illnesses include heat stroke, heat exhaustion, heat cramps, and heat rash conditions. Without immediate and correct treatments, patients may suffer from brain damage or other organ failures, and even death. Heat exposure can also cause fatigue and then lead to other construction accidents. It is crucial to raise awareness of heat-related illnesses and equip people with the means and methods to prevent them from happening. This study aims to assess the current status of people’s knowledge of heat-related illnesses using an online survey questionnaire. The results suggest that further heat-related training should put more focus on the training topics of heat-related illnesses prevention strategies, first-aid, and symptom identifications. The participants whose work is directly related to safety have a better understanding of heat-related illness symptoms than the rest. The results also indicate that the participants’ knowledge of heat-related illness prevention is related to their age and whether they had participated in heat-related training. The findings of this research can help with the development of future heat-related illnesses training and facilitate construction companies to improve their current safety culture and practices.