Objective  Appendiceal mucinous tumors are rare, which can cause peritoneal metastasis and form pseudomyxoma peritonei (PMP). Currently, cytoreductive surgery (CRS) and hyperthermic intraperitoneal chemotherapy (HIPEC) are usually adopted for treating appendiceal mucinous tumors. However, correlations between histological type, peritoneal carcinomatosis index (PCI) score, and completeness of cytoreduction (CC) score and prognosis remain unclear. Thus, this study aimed to investigate the effect of histological classification, PCI score, and CC score on the prognosis of patients with appendiceal mucinous tumors with peritoneal metastasis.  Methods  Data on outcomes of appendiceal mucinous tumors treated with CRS combined with HIPEC at Chongqing University Cancer Hospital from March 2009 to January 2019 were reviewed. According to the digestive system classification criteria (5th edition, 2019) recommended by the World Health Organization (WHO) and the Peritoneal Surface Oncology Group International (PSOGI), a Cox proportional hazard model was used to determine the effect of tissue classification, PCI score, and CC score on the progression-free survival (PFS) of the patients.   Results  In total, 48 patients were treated with CRS combined with HIPEC. In the univariate Cox regression analysis, PCI score, CC score, primary histological type, and peritoneal histological type had statistical effects on PFS ( P<0.05). Compared with PCI score≤ 10, the risk ratio of 20-30 scores was 10.38. Compared with 0, the risk ratios of CC score 1 and 3 were 4.26 and 14.74, respectively. The risk ratio of signet-ring cell carcinoma in patients with the primary histological type was 9.81 compared with that of low-grade mucinous tumors, while the risk ratio of high-grade mucinous adenocarcinoma was 14.35 compared with that of acellular mucin tumors. In the multivariate Cox regression analysis, only primary histological type had a significant effect on PFS ( P<0.05). The risk ratio of signet-ring cell carcinoma was 110.79 in patients with the primary histological type compared with that of low-grade mucinous tumors.  Conclusions  Standardizing the histological classification of primary and peritoneal lesions for the prognostic evaluation of patients with appendiceal mucinous tumors and peritoneal pseudomyxoma treated with CRS combined with HIPEC is of great importance. The histological malignant degree of peritoneal lesions was positively correlated with that of primary lesions, and the higher malignancy of primary and peritoneal lesions was attributed to a worse patient prognosis. Compared with the histological type of peritoneal lesions, that of primary lesions was more closely related to patient prognosis and was therefore a better predictor. Additionally, patient prognosis was related to CC and PCI scores of patients treated with CRS, and higher PCI and CC scores were attributed to a worse patient prognosis. Therefore, to treat patients with CRS combined with HIPEC, we should standardize the histological classification of primary and peritoneal lesions, reduce the tumor completely, and normatively administer HIPEC for the patients’ full benefit.