string host;//服務(wù)端url，如ftp://127.0.0.1
string username;//用戶名，如root
string password;//密碼，如123
string remoteFilePath;//下載文件路徑，如file.db
string savePath;//保存文件路徑，如D:\file.db

FtpWebRequest request = (FtpWebRequest)WebRequest.Create(new Uri(new Uri(host), remoteFilePath));
request.Method = WebRequestMethods.Ftp.DownloadFile;
if (string.IsNullOrEmpty(username) && string.IsNullOrEmpty(password))
    request.Credentials = null;
else
    request.Credentials = new NetworkCredential(username, password);
using (FtpWebResponse response = (FtpWebResponse)request.GetResponse())
using (Stream responseStream = response.GetResponseStream())
using (FileStream fileStream = new FileStream(savePath, FileMode.Create, FileAccess.ReadWrite, FileShare.ReadWrite))
{
    responseStream.CopyTo(fileStream);
}