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Background Frailty can increase the risk of negative health-related outcomes in older adults. Protein supplementation may be an effective way to improve frailty, but there is disagreement about its effects on frailty. Objective To systematically evaluate the effects of protein supplementation on muscle mass, strength, and physical function in frail/pre-frail older adults. Methods Electronic databases of CNKI, Wanfang Data, CQVIP, PubMed, Embase, Web of Science, Cochrane Library, CINAHL and Medline were retrieved for randomized controlled trials (RCTs) of the effects of protein supplementation on muscle mass, strength and physical function in frail/pre-frail older adults published from inception to June 2022. After literature screening, the quality of eligible RCTs was evaluated, and from which relevant data were extracted. RevMan 5.4 was performed to explore the effects of protein supplementation on muscle mass, muscle strength and physical function in frail/pre-frail older adults. And for the outcome indicator of muscle strength (grip strength) , due to large amount of reported literature, this study will be based on the amount of protein supplementation (<30 g/d subgroup and≥30 g/d subgroup) , frailty status (pre-frailty subgroup, frailty subgroup, frailty and pre-frailty subgroup) , frailty assessment tool 〔frailty phenotype assessment tool (FP) subgroup and non-FP subgroup〕, population (Asian subgroup and European subgroup) , and mean age (70-<75 years subgroup, 75-<80 years subgroup, and 80-<85 years subgroup) for subgroup analysis to further explore the effect of protein supplementation on grip strength in different subgroups. Results A total of 12 RCTs were included (2 literatures for pre-frailty, 3 literatures for frailty, 7 literatures for frailty and pre-frailty) , with a total of 833 older adults (422 in the protein supplementation group and 411 in the control group) . Meta-analysis results showed that protein supplementation improved gait speed in frail/pre-frail older adults〔MD=0.03, 95%CI (0, 0.06) , P=0.05〕, but in improving muscle mass (appendicular lean mass) , muscle strength (grip strength) , other physical functions (assessment results of balance test, the timed up and go test, Short Physical Performance Battery) and frailty scores, the differences were not statistically significant (P>0.05) . The results of subgroup analysis showed that the effect of protein supplementation on the grip strength of the Asian population subgroup was significantly different from that of the European population subgroup in between-group comparisons (χ2=5.76, P=0.02) . Conclusion Protein supplementation may improve gait speed in frail/pre-frail older adults, but it does not show a significant advantage in improving their muscle mass, muscle strength and other physical functions. It is recommended to further investigate the effects of longer durations of supplementation, different types of protein supplemented, different amounts of supplementation and different regional populations on older adults with different frailty states, in order to find the best pattern of protein supplementation and provide a more sufficient evidence-based basis for frailty management.