Objective To investigate the variation patterns of rectal temperature and heart rate during outdoor heat acclimatization training, and to identify sensitive physiological indicators for monitoring the training load. Methods A total of 110 healthy adult male volunteers were recruited and assigned to high endurance (HE) group (n=60) or qualified endurance (QE) group (n=50) based on their 3-km running performance. The 2 groups received 7 and 12 d of outdoor heat acclimatization training, respectively, which mainly consisted of 5-km loaded fast walking. Rectal temperature and heart rate were monitored in real time throughout the training, and the variation patterns were analyzed. Results Training intensity was adjusted daily according to the wet-bulb globe temperature outdoors. In the HE group, mean rectal temperature and maximum heart rate reached heat acclimatization on days 6 and 5, respectively; in the QE group, both indicators reached heat acclimatization on day 8. During the first 5-km loaded fast walking session, both heart rate and rectal temperature showed a continuous upward trend, with heart rate changing earlier and more rapidly than rectal temperature. During the 10-min rest interval between the 2 sessions, heart rate rapidly decreased, while rectal temperature paradoxically continued to rise. During the second 5-km loaded fast walking session, heart rate continued to rise, whereas rectal temperature initially decreased, then increased and remained consistently elevated. Using rectal temperature ≥38.00 ℃ as the criterion for training compliance, more than 88% of the subjects achieved the desired heat acclimatization training effect, and acquiring heat acclimatization required a daily compliant training duration ＞24 min. Conclusion The continuous variation of rectal temperature accurately reflects the physiological responses of the human body during heat acclimatization training. It is recommended as a monitoring indicator for outdoor heat acclimatization training load. In contrast, heart rate primarily reflects exercise intensity and is not recommended as a monitoring indicator for heat acclimatization training load.