Objective To explore the effect of one-time instant root canal filling with prepared warm gutta-percha on chronic apical periodontitis (CAP). Methods 186 patients with CAP were randomly divided into study group (93 cases) and control group (93 cases). The patients in the study group were treated with one-time instant root canal filling with prepared warm gutta-percha, while the other patients in the control group were treated with cold gutta-percha filling. The effects of root canal filling, pain in 1 week after operation and curative effects in 1 year after operation were compared between the two groups. Results The excellent rate of root canal filling and total effective rate in 1 year after operation of the study group were all higher than those of the control group, the incidence rate of pain in 1 week after operation and visual analogue scale (VAS) score of the study group were all lower than those of the control group (P<0.05). Conclusion One-time instant root canal filling with prepared warm gutta-percha which has better filling effect and higher curative effect than cold gutta-percha filling, can reduce postoperative pain for CAP patients.