Dynamic adaptive streaming over HTTP (DASH) has emerged as an efficient technology for video streaming. For a DASH system, a most common case is that a limited server bandwidth is competed by multiusers. In order to improve user quality of experience (QoE) and guarantee fairness, we propose to use the game theory in a proxy server to allocate the bandwidth collaboratively for multiusers. By taking user buffer length, received video bit rates, video qualities, etc., into account, the bandwidth allocation problem is formulated as a cooperative bargaining problem and the Nash bargaining solution (NBS) is obtained by convex optimization. The requested bit rate of users will be rewritten as the proxy calculated bit rate (i.e., NBS) when the user requested bit rate is larger. Experimental results demonstrate that user QoE and fairness can be improved significantly, i.e., the delay frequency and duration are smaller, and the received video qualities are higher and more stable, when comparing the proposed method with existing methods.