Objective To evaluate the effect of sufentanil combined with dexmedetomidine on postoperative analgesic effect and respiratory depression in patients undergoing aradical surgery of gastric cancer/colorectal cancer. Methods From August 2018 to August 2020, 80 patients admitted to our hospital for aradical surgery of gastric cancer/colorectal cancer were included in the study. The patients were divided into an observation group and a control group with 40 cases in each group by random number table method. Sufentanil was used as the analgesic agent for the control group, and dexmedetomidine was additionally used for the observation group. The analgesic and sedative effects, mean arterial pressure （MAP）, blood oxygen saturation （SpO₂）, the occurrence of adverse reactions, as well as the dosage of sufentanil, the dosage of PCIA, the number of additional analgesics, and the number of analgesic pump presses 24 h after surgery were analyzed between the two groups. Results There was no significant difference in VAS and Ramsay scores between the two groups before surgery （P>0.05）. At 12 h, 24 h, 36 h, and 48 h after surgery, the VAS scores of the observation group were lower than those of the control group, while the Ramsay scores were higher （P<0.05）. There were no significant differences in MAP and SpO₂ between the two groups （P>0.05）. At 12 h, 24 h, 36 h, and 48 h after operation, the MAP of the observation group was lower than that of the control group （P<0.05）. The incidence rate of nausea and vomiting in the observation group was lower than that in the control group （P<0.05）. There was no respiratory depression in the two groups. There were no significant differences in the incidence rates of chills, pruritus, bradycardia, and dizziness between the two groups （P>0.05）. The dosage of sufentanil, the dosage of PCIA, the number of additional analgesics, and the number of analgesic pump presses in the observation group were all lower than those in the control group 24 h after operation （all P<0.05）. Conclusion Sufentanil combined with dexmedetomidine has a better postoperative analgesic effect on patients with aradical surgery of gastric cancer/colorectal cancer, does not cause respiratory depression, and can reduce the use of analgesic drugs and the number of analgesic pump presses.