Objective To compare the effects of three measurement methods on the insertion depth of nasopharyngeal temperature probes in surgical patients and to identify a simple and accurate method for determining the optimal insertion length. Methods A total of 147 patients undergoing endotracheal intubation under general anesthesia were selected using a convenience sampling method and randomly assigned into three groups (n = 49 each) according to a random number table. Group A (fiberoptic bronchoscopy group) underwent nasopharyngeal temperature probe insertion under direct visualization using a fiberoptic bronchoscope. In group B (curved soft ruler group), the curved surface distance from the outermost point of the nasal ala to the lowest point of the ipsilateral earlobe was measured using a medical-grade flexible plastic ruler, and the probe was inserted according to the measured length. In group C (vertical steel ruler group), the vertical distance between the same anatomical landmarks was measured using an L-shaped steel ruler. The insertion depth of the nasopharyngeal temperature probe and patients′core body temperature were compared among the three groups. Results There were statistically significant differences in the insertion depth of the nasopharyngeal temperature probe among the three groups (P<0.05). Significant differences were observed between Group B and Group A, as well as between Group B and Group C (all P<0.05). Core body temperature measurements at different time points also showed statistically significant differences among the three groups (P<0.05). Compared with Group A, Group B showed significant differences at five time points, and compared with Group C, significant differences were observed at three time points (all P<0.05). Conclusion The fiberoptic bronchoscopy-guided method and the vertical measurement method resulted in probe insertion depths closer to the theoretical value and provided temperature readings more consistent with core body temperature. The vertical measurement method was simpler to perform and may be more practical for clinical application.