To solve the high consumption problem of fast video Shot Boundary Detection (SBD), an improved Shot Boundary Detection (SBD) method was proposed based on video pre-processing. Adaptive threshold was taken to select the candidate segments which may contain shot boundaries, the beginning frame was detected by comparing the similarity value between the first frame and the rest frames in the candidate segments, and then cut was detected immediately. Gradual transition would be detected if no cut was detected. Candidate segments had to be adjusted to ensure the whole transition was located in the same segment. Ending frame was confirmed by comparing the similarity value between the beginning frame and the rest frames in segment. Experimental results demonstrate that the proposed algorithm achieves an accuracy above 90% and the time cost is reduced 15.6%-30.2% compared with inverted triangle pattern matching method. The proposed algorithm satisfies the need of accuracy and improves detection speed compared with the traditional methods which need detection for both cut and gradual transition.