This study focused on the effect of examinees' ability levels on the relationship between Reflective-Impulsive (RI) cognitive style and item response time in computerized adaptive testing (CAT). The total of 56 students majoring in Educational Technology from Shandong Normal University participated in this study, and their RI cognitive styles were diagnosed using the Matching Familiar Figures Test-20 (MFFT-20). Examinees' ability values and average item response time were recorded by the computerized adaptive testing system. Then mediation analysis was implemented and the findings revealed that there was direct and indirect effect between RI cognitive style and item response time in CAT. What is more, RI cognitive style also directly affected the ability levels, and then the ability levels impacted on item response time. So, examinee's ability level was partly a mediator between RI cognitive style and item response time. Furthermore, RI cognitive style of the examinees might also be diagnosed according to ability values and average item response time. The relevant research and implications were further discussed.