Objective To explore the longitudinal association between postoperative pain and frailty in elderly patients who have undergone total knee arthroplasty (TKA), and to provide a reference for developing targeted pain management strategies to reduce the risk of frailty.Methods A convenience sample of 235 elderly patients undergoing TKA were enrolled. General patient data were collected one day before surgery. Pain and frailty levels were assessed at one week, one month, three months, and six months postoperatively. Group-based trajectory modeling was used to identify distinct trajectories of postoperative pain. A generalized estimating equation model was applied to analyze the longitudinal association between postoperative pain and frailty. Results A total of 203 patients completed the follow-up. Three distinct postoperative pain trajectories were identified: mild pain-rapid decline group (59.6%), moderate pain-gradual decline group (32.0%), and severe pain-slow decline group (8.4%). After adjusting for confounding factors, postoperative pain trajectories were significantly associated with frailty status (P<0.05). Significant gradient differences existed in the marginal effects of frailty across different pain trajectory groups at each time point. At the same time point, the marginal effect of frailty in the severe pain-slow decline group was the highest, followed by the moderate pain-gradual decline group and the mild pain-rapid decline group (P<0.05). Conclusion Postoperative pain trajectories are heterogeneous in elderly patients after TKA and are closely associated with frailty status. Healthcare professionals should prioritize postoperative pain management and develop tailored interventions for different trajectory groups to prevent or delay frailty in elderly patients after TKA.