Background Frailty serves as a significant precursor to falls, disability, and mortality. Epidemiological evidence examining the impact of working hours on frailty remains scarce. Objective To explore the prevalence of frailty and the relationship between frailty and working hours among full-time middle-aged and older workers in China. Methods Data were derived from the 2020 China Health and Retirement Longitudinal Study (CHARLS). The study included full-time workers aged 45 years and above with a weekly working duration exceeding 35 h. Frailty was assessed using the Frailty Index (FI). First, the dose-response relationship between working hours and FI was explored using a generalized additive model (GAM). Second, univariate analyses were performed using t-tests, χ² tests, and the Jonckheere–Terpstra trend test. Restricted cubic splines (RCS) were introduced for modeling. Based on ordinal logistic regression models, covariates were adjusted sequentially to examine whether differences in frailty prevalence existed across different weekly working hour categories. Finally, subgroup analyses were performed. Results Among the 5504 included participants, the mean age was (56.41±7.30) years, with 2304 being female (41.86%). A total of 807 participants (14.66%) were identified as frail, and 2 686 participants (48.80%) were pre-frail. After standardization for gender and age according to the seventh national census data, the standardized prevalence of frailty and prefrailty were 18.89% and 47.25%, respectively. Significant higher proportions of frail individuals were found among those in non-managerial (73.73%), agricultural (64.06%), and non-employed (65.68%) occupations. Long working hours were prevalent: 2596 (47.17%) workers worked 41–60 h per week, 1649 (29.96%) worked >60 h per week, and only 1259 (22.87%) worked 35–40 h per week. GAM analysis showed a non-linear relationship between weekly working hours and FI. Restricted cubic spline analysis showed a statistically significant association between weekly working hours and frailty status (P=0.022). Logistic regression analysis revealed that individuals working more than 60 h per week had a 23% higher odds of frailty compared to those working 41–60 h (OR=1.23, 95%CI: 1.09, 1.38, P<0.01), while those working 35–40 h per week showed a 15% higher odds of frailty (OR=1.15, 95%CI: 1.01, 1.31, P=0.034). The subgroup analysis revealed that the positive association between working more than 60 h per week and frailty remained generally consistent across sex, age, urban/rural residence, occupational attributes, industry, or employment type, although the association did not reach statistical significance among non-rural residents and individuals aged 60 years and older. Conclusion The prevalence of frailty among full-time middle-aged and older labor force in China is relatively high, and the odds of frailty increases significantly among those who work more than 60 h per week. It is recommended to implement frailty prevention and management strategies targeting long-working-hour workers and to prioritize the impact of working duration on frailty.