static void Main(string[] args)
        {
            string targetSiteURL = @"https://xxx.sharepoint.com/sites/xxx";
            string sourcePath = "C:\\Xie\\Test\\";

            Console.WriteLine("Enter Your Username Please  ------- ");
            string username = Console.ReadLine();

            Console.WriteLine("Enter Your Password Please  ------- ");
            SecureString password = GetPasswordOfYourSite();

            SharePointOnlineCredentials onlineCredentials = new SharePointOnlineCredentials(username, password);

            using (ClientContext ctx = new ClientContext(targetSiteURL))
            {
                ctx.Credentials = onlineCredentials;
                Web web = ctx.Web;
                ctx.Load(web);

                List targetList = ctx.Web.Lists.GetByTitle("TestFolder");

                FolderCollection oFolderCollection = targetList.RootFolder.Folders;

                ctx.Load(oFolderCollection);
                ctx.ExecuteQuery();

                foreach (Folder oFolder in oFolderCollection)
                {
                    Console.WriteLine("Folder Name: " + oFolder.Name + " Folder URL: " + oFolder.ServerRelativeUrl);

                    Folder folder = web.GetFolderByServerRelativeUrl(oFolder.ServerRelativeUrl);
                    ctx.Load(folder);

                    DirectoryInfo directory = new DirectoryInfo(sourcePath);
                    FileInfo[] files = directory.GetFiles("*.*");
                    foreach (FileInfo file in files)
                    {
                        var filePath = sourcePath + file.Name;
                        FileCreationInformation newFile = new FileCreationInformation();
                        newFile.Content = System.IO.File.ReadAllBytes(sourcePath + file.Name);
                        newFile.Url = file.Name;
                        newFile.Overwrite = true;
                        Microsoft.SharePoint.Client.File newspFile = folder.Files.Add(newFile);
                    }
                    ctx.ExecuteQuery();
                }

                Console.ReadLine();
            }
        }

        private static SecureString GetPasswordOfYourSite()
        {
            ConsoleKeyInfo info;
            //Get the user's password as a SecureString  
            SecureString securePassword = new SecureString();
            do
            {
                info = Console.ReadKey(true);
                if (info.Key != ConsoleKey.Enter)
                {
                    securePassword.AppendChar(info.KeyChar);
                }
            }
            while (info.Key != ConsoleKey.Enter);
            return securePassword;
        }