找了很多資料，最後從論壇上找到的解決方案，這裡貼一下幾處關鍵程式碼

<action name="filedownload" class="fileDownloadAction">
    <result name="success" type="stream">
        <param name="contentType">application/octet-stream;charset=ISO8859-1</param> //注意這裡的charset要寫
        <param name="contentDisposition">attachment;filename="${fileName}"</param>
        <param name="inputName">downloadFile</param>
    </result>
</action>
 

ServletActionContext.getRequest().setAttribute("report",URLEncoder.encode(“我是中文.txt”, "utf-8"));

得到的是字串。將此字串傳給url：

<a href="student/filedownload?fileName=${report}">

public InputStream getDownloadFile() throws Exception {
    fileName = new String(fileName.getBytes("ISO-8859-1")); //將編碼後的中文名解碼，得到fileName為“我是中文.txt”
 

public String getFileName() throws Exception {
    return new String(fileName.getBytes(), "ISO8859-1");
}