Objective To assess the diagnostic value of multi slice spiral computed tomography (MSCT) thin layer adding three dimensional reconstruction image for localized peritonitis and the reason caused localized peritonitis. Methods Abdominal MSCT data of 158 patients suspected localized peritonitis were retrospectively analyzed. All patients experienced abdominal unenhanced scan and the original image has been reconstructed with thin layer. All image data were divided into two groups, that were group A (original image) and group B (thin layer adding three dimensional reconstruction image). The sensitivity, specificity, positive predictive value, negative predictive value, and accuracy of group A and group B in diagnostic of localized peritonitis were analyzed. Results One hundred and twelve patients with localized peritonitis were confirmed by surgical operation or follow-up, in which 101 had obvious reason caused localized peritonitis. 15 patients have been missed diagnosis by group A and 25 patients could not confirmed the reason caused localized peritonitis in group A. However, there were no patients missed diagnosis in group B and the reason caused localized peritonitis of all patients has been detected by group B. The sensitivity, specificity, positive predictive value, negative predictive value, and accuracy of two group were 77.7% vs.100%，100% vs. 100%，100% vs. 100%，64.8% vs. 100% and 84.2% vs. 100%，respectively. Conclusion MSCT thin layer adding three dimensional reconstruction image can increase the diagnostic accuracy of localized peritonitis and the reason caused localized peritonitis.