Objective Explore compounding 5 μg sufentanil effective dose of remiazolam toluenesulfonate when is used for painless colonoscopy in elderly patients. Methods A total of 50 elderly patients were included. The first patient was given remimazolam 0.15 mg/kg. The concentration of remimazolam of the second patient was determined by Biased Coin Design (BCD) according to the body moving response to colonoscopy examination in the first patient. The dose of next patient was determined using the same method. According to the observation results, the effective dose of remimazolam was analyzed and calculated [median effect concentration (EC50) and effective concentration 90 (EC90)]. Record the patient's physical reaction during colonoscopy. Adverse reactions such as injection pain, vomiting, hypoxemia, bradycardia, pruritus, urinary retention, and allergy to test drugs were recorded. Results Combined with 5 μg sufentanil, the EC50 and EC90 of remimazolam required for elderly painless colonoscopy were 0.146 (0.027, 0.155) mg/kg and 0.163 (0.149, 0.317) mg/kg, respectively. There was two patients (2% each) suffered from injection pain and hypoxia, respectively while 3 patients (6%) suffered postoperative vomiting. No other adverse reactions were observed. Conclusions The EC50 and EC90 of remimazolam required for elderly painless colonoscopy was 0.146 (0.027, 0.155) mg/kg and 0.163 (0.149, 0.317) mg/kg when combined with 5 μg sufentanil.