Nursing graduates experience "reality shock" upon entering the workplace as well as face challenges and stress during their transition from nursing student to qualified nurse. The high turnover rate of new graduate nurses not only increases an organization's human resources costs but also results in a shortage of nurses. In particular, a poor experience during the transition from nursing student to qualified nurse can lead to significant turnover. A three-year longitudinal study was conducted to assess the effectiveness of one-on-one mentorship program in reducing the turnover rate of nurses in China. A total of 199 new graduate nurses, recruited in 2013, were considered the control group and received a basic preceptorship, and 239 nurses in 2014 were considered the experimental group, for which a one-on-one mentorship program was implemented. Propensity-score-matching analysis was conducted to adjust the baseline of the two groups, and survival analysis was performed to compare the two groups. The findings showed that the turnover rates for the experimental group were 3.77%, 3.48%, and 8.11% as compared to 14.07%, 9.36%, and 14.19% for the control group at the end of the first three years, respectively. The survival curves of the two groups were significantly different (p < 0.001). The turnover rate for the first year in the experimental group was significantly lower than that for the control group, but the rates in the second and third years were not different. The results indicate that a one-on-one mentorship program is beneficial for the retention of new graduate nurses, particularly during the first year.