Objective To analyze the differences of ultrasonographic features between acute appendicitis and normal appendix within critical outer diameter, and to explore the significance of these ultrasonographic features in the diagnosis of acute appendicitis in children. Methods From January 2019 to May 2023, children suspected of acute appendicitis and received ultrasound examination in our hospital were continuously collected and divided into confirmed appendicitis group and excluded appendicitis group according to the final clinical diagnosis results. Compared the differences of ultrasonographic features between the two groups of cases, and found out the ultrasonographic features which were helpful to the differential diagnosis of acute appendicitis and normal appendix. Results There were significant differences between the two groups in the ultrasonographic features of periappendicular mesenteric and omental thickening, appendicular fecalith and periappendicular effusion (p<0.05). Periappendiceal mesenteric and omental thickening had good and stable diagnostic efficacy in the differential diagnosed between acute appendicitis and normal appendix within the critical outer diameter(AUC=0.906, 95% CI 0.855—0.957), the sensitivity was 100% (95%CI 96.4%—100%), and the specificity was 81% (95%CI 71.9%-88.2%). Conclusion Ultrasound could be used effectively in the differential diagnosis of acute appendicitis and normal appendix within the critical outer diameter, and the ultrasonographic features of periappendicular mesenteric and omental thickening had great value in differential diagnosis.