The high grades in postgraduates entrance examination are the stepping stone to improve the professional competition in China, which makes the critical influence factors to the initial examination of postgraduates as a hot topic. In order to measure the correlation between the academic performance of undergraduate students and the probability of successful postgraduate entrance examination, and find some effective ways to improve the success rate in the postgraduate entrance examination, we design regression discontinuity model to search the causal correlation between the preliminary entrance exam and CET-6 scores of candidates. Global parameter estimation and local non-parameter estimation were estimated respectively to improve the accuracy of model fitting. In addition, the validity and robustness of the experiment were tested by sensitivity analysis. The results show that the students who get better grades in college are more likely to have a positive effect in the preliminary entrance examination, and make the success rate of postgraduate entrance examination higher.