This paper mainly studies the technology of multi-thread downloading in browser, which aims to solve the problem of low efficiency of single thread downloading and over reliance on the target server. Based on HTML5 Web Workers technology, we propose and implement a novel multi-thread downloading technology which runs in the browser. Though segmenting file to chunks, we download a file from multiple sources. We use ArrayBuffer array and Blob objects to merge the file fragments in the browser. The results show that this method is superior to the single thread downloading technology in the large file downloading, large network delay or high packet loss rate.