Objective To investigate the effect of different dilution methods of replacement fluid on cardiopulmonary bypass lifespan and small molecule clearance rate in continuous renal replacement therapy (CRRT) without anticoagulation. Methods Patients who needed to undergo CRRT in West China Hospital of Sichuan University between December 2019 and June 2020 were selected in this prospective cohort study. Continuous veno-venous hemodiafiltration treatment mode was applied, with three different dilution methods, namely pre-dilution, post-dilution, and pre- and post-dilution. The lifespan of cardiopulmonary bypass was recorded, and blood samples were taken to detect creatinine, blood urea nitrogen, etc. Only the lifespan of the first tube of each patient was included. Results A total of 74 patients and pipelines were included, including 18 pre-dilution cases, 34 pre- and post-dilution cases, and 22 post-dilution cases. There was no significant difference in the baseline gender, age, catheterization method, or coagulation function among the three groups of patients (P>0.05). The service life of the circulation pipeline was (32.67±17.42) h in the pre-dilution group and (30.32±16.77) h in the post-dilution group, respectively, and the difference was not statistically significant (P<0.05); the service life of the circulation pipeline was (44.15±21.48) h in the pre- and post-dilution group, which was statistically different from those in the other two groups (P<0.05). There was no statistically significant difference in the decrease of blood urea nitrogen or serum creatinine among the three groups when treated with the three different dilution methods (P>0.05). Conclusion On the premise of not affecting the clearance rate of small molecules, compared with pre-dilution and post-dilution, pre- and post-dilution can prolong the service life of extracorporeal circulation pipelines and filters, and has certain promotion value in clinical practice.