program Mdiform;

uses
Forms,
UMdiform in 'UMdiform.pas' {Mainform},
UDM in 'UDM.pas' {GlobalDM: TDataModule},
UFun in 'UFun.pas';

{$R *.res}

begin
Application.Initialize;
Application.CreateForm(TMainform, Mainform);
Application.Run;
end.
===============================================================================================

主窗体代码：
unit UMdiform;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, Udm,StdCtrls;
type

TMainform = class(TForm)
Button1: TButton;
procedure Button1Click(Sender: TObject);
procedure FormCreate(Sender: TObject);
private
{ Private declarations }
public
MyHandle:HWND;
{ Public declarations }
end;

var
Mainform: TMainform;

type
T_ShowTestMng=function (var adm:TMainform) : Boolean; StdCall;

implementation
{$R *.dfm}

procedure TMainform.Button1Click(Sender: TObject);
var
Lib_ :THandle;
_ShowTestMng :T_ShowTestMng;
begin
Lib_:=LoadLibrary(pchar('MdiChild.dll'));
try
@_ShowTestMng:=GetProcAddress(Lib_,'_ShowTestMng');
if not(@_ShowTestMng=nil) then
_ShowTestMng(Mainform);
finally
FreeLibrary(Lib_);
end;
end;

procedure TMainform.FormCreate(Sender: TObject);
begin
MyHandle:=Application.Handle;
end;

end.
==============================================================================================

子窗体DLL代码：
library MdiChild;

uses
ShareMem,
UMdiform, //此单元为父窗体单元，在顶目设置中我已经设置了搜索此单元在路径。
Forms,
SysUtils,
Classes,
UChild in 'UChild.pas' {FrmChild};//FrmChild子窗体的FormStyle属性为FsMDIChild

{$R *.res}
function _ShowTestMng(var adm:TMainform) : Boolean; StdCall;
begin
result:=true;
Application.Handle:=adm.MyHandle;
Application.CreateForm(TFrmChild,FrmChild); //程序就出错在此：出错原因是：Cannot create form. No MDI Forms are currently active.
FrmChild.Show;
end;

exports
_ShowTestMng;
end.