Many outpatients who inadequately prepared for the procedure were cancelled on the day of the examination for various reasons. The aim of study was to investigate whether short message service (SMS) can improve patients' compliance and reduce cancellation rates. Outpatients scheduled for sedation gastrointestinal endoscopy were randomly assigned to mobile phone SMS group or control group. Patients in the control group received a leaflet on preparation instructions, while patients in the SMS group received SMS reminders after making an appointment. A total of 1786 patients were analyzed. There was a significant reduction in the rate of cancellations for patients in the SMS group (4.8%) compared with patients in the control group (8.0%) (P<0.001). Patients in the SMS group were 40% less likely to be cancelled by medical staff than patients in the control group. The compliance score of the two groups based on demographic and clinic characteristic distribution showed that for both male and female patients, the compliance score was higher in the SMS group than that in the control group (P=0.023, P<0.001, respectively). Additionally, the compliance score was also significantly higher in the SMS group among patients who were under 50 years old, less than an undergraduate education level, experiencing their first time for procedure, or whose procedures were gastroscopy, waiting time was between 4 and 15 days, and schedules were in morning (P≤0.032). SMS reminders can be considered a complement to conventional preparation instructions, which could help improve the compliance of outpatients and reduce the rate of cancellations.