Due to the inherent randomness of bandwidth, the existing rate adaptation algorithms based on Dynamic Adaptive Streaming over Hyper Text Transfer Protocol (DASH) fail to make a balance between playback fluency and video quality. Concerning the above problem, a State machine-based DASH (SDASH) algorithm was proposed to analyze and control the rate switching process. Firstly, the influence factors of client's Quality of Experience (QoE) were fully considered and numerically analyzed. Secondly, six bitrate states were proposed according to the influence factors, and the relations between the video bitrate and the changes in influence factors' values were used as the state transition conditions. Finally, while the playback buffer and the bitrate deviation ratio satisfying the threshold condition, the video bitrate was switched to a rate level which has the relatively optimum overall performance of playback fluency and video quality. The experimental results demonstrate that the proposed algorithm can not only improve the video bitrate compared with the fuzzy-based DASH adaptation algorithm but also avoid bitrate plunging, thus reaching a balance between playback fluency and video quality, and leading to an improvement of QoE.