Dynamic adaptive streaming over HTTP (DASH) has been widely used in the video transmission. With the limited client buffer and varying network, this paper proposes a new adaptive bitrate switching algorithm based on the client buffer. The proposed algorithm divides the client buffer into several buffer levels and uses sliding window to analyze the download time of recent video segments in order to maintain the media buffer size in the balance interval. The algorithm uses a conservative strategy when switching to bit rates upwards, and when switching down, the logistic equation is used to determine the magnitude of the bit rate switching. In addition, this paper also proposed a fast start strategy, in which download rate could match the network bandwidth quickly after the system starts. Experiment results show that the proposed algorithm can make full use of bandwidth and buffer ,and reduce the number of rate switching in complex network environment.