// Win32 Constants

        private const int GWL_STYLE = -;

        private const int GWL_EXSTYLE = -;

        private const int WS_BORDER = 0x00800000;

        private const int WS_EX_CLIENTEDGE = 0x00000200;

        private const uint SWP_NOSIZE = 0x0001;

        private const uint SWP_NOMOVE = 0x0002;

        private const uint SWP_NOZORDER = 0x0004;

        private const uint SWP_NOACTIVATE = 0x0010;

        private const uint SWP_FRAMECHANGED = 0x0020;

        private const uint SWP_NOOWNERZORDER = 0x0200;

        // Win32 方法

        [DllImport("user32.dll", CharSet = CharSet.Auto)]

        private static extern int GetWindowLong(IntPtr hWnd, int Index);

        [DllImport("user32.dll", CharSet = CharSet.Auto)]

        private static extern int SetWindowLong(IntPtr hWnd, int Index, int Value);

        [DllImport("user32.dll", ExactSpelling = true)]

        private static extern int SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter,

            int X, int Y, int cx, int cy, uint uFlags);

获取到API后在Form_Load事件中加入如下代码  

  //获取mdi客户区

            for (int i = ; i < this.Controls.Count; i++)

            {

                var mdiClientForm = this.Controls[i] as MdiClient;

                if (mdiClientForm == null) continue;

                // 找到了mdi客户区

                // 取得客户区的边框

                int style = GetWindowLong(mdiClientForm.Handle, GWL_STYLE);

                int exStyle = GetWindowLong(mdiClientForm.Handle, GWL_EXSTYLE);

                style &= ~WS_BORDER;

                exStyle &= ~WS_EX_CLIENTEDGE;

                // 调用win32设定样式

                SetWindowLong(mdiClientForm.Handle, GWL_STYLE, style);

                SetWindowLong(mdiClientForm.Handle, GWL_EXSTYLE, exStyle);

                // 更新客户区

                SetWindowPos(mdiClientForm.Handle, IntPtr.Zero, , , , ,

                    SWP_NOACTIVATE | SWP_NOMOVE | SWP_NOSIZE | SWP_NOZORDER |

                    SWP_NOOWNERZORDER | SWP_FRAMECHANGED);

                UpdateStyles();

                break;

            }

重新运行程序，OK