Objective <p>This study aimed to identify independent predictors of under-triage among patients with non-traumatic acute abdominal pain (NAAP) and quantify their impact on emergency department length of stay (EDLOS).</p> Methods <p>A multicenter retrospective study was conducted at three tertiary hospitals in China from December 2023 to May 2024, enrolling 769 patients with NAAP. Data collected included patient demographics, environmental factors, nurse characteristics, and clinical presentations. Binary logistic regression was employed to identify risk factors for under-triage, and multivariate linear regression was used to assess its association with EDLOS.</p> Results <p>The incidence of under-triage was 16.38% (126/769). Multivariate analysis revealed that a history of hyperlipidemia (OR = 7.944), scleral icterus (OR = 5.731), persistent pain (OR = 4.116), and advanced age (OR = 2.447) were significant independent risk factors for under-triage (all <i>P</i> &lt; 0.05). Conversely, higher nursing seniority (Senior Nurse: OR = 0.295; Charge Nurse: OR = 0.311) served as a protective factor. Furthermore, under-triage independently predicted operational inefficiency, prolonging EDLOS by approximately 310&#xa0;min (B = 309.765, <i>P</i> &lt; 0.001) after adjustment for confounders.</p> Conclusion <p>This multicenter study demonstrates that under-triage in NAAP is associated with patient age, history of hyperlipidemia, higher nurse professional titles, and abdominal pain characteristics. Under-triage significantly prolongs EDLOS. These findings provide a foundation for understanding and mitigating under-triage through competency-based training and risk-stratified assessment protocols in emergency departments.</p>