To detect early prostate cancer, physicians usually apply the color Doppler ultrasound imaging to visualize prostate. Since color Doppler ultrasound imaging provides blood flow information, differentiating the malignancy from benign prostate diseases by inspecting Doppler spectrum is not difficult. However, it is hand-held, time-consuming, and only limited in a small and static region, which might not represent the real status of the whole prostate. Therefore, this paper proposed a dynamic approach to estimate resistive index of blood flow in color Doppler ultrasound images. Experimental results show dynamic resistive index is better than conventional resistive index, and comparable with PSA test in diagnosing prostate cancer, indicating high performance of the proposed approach, which also reveals feasible and effective of it.