private void button2_Click(object sender, EventArgs e)
        {
            
            bool have = false;
            foreach (TabPage a in tabControl1.TabPages)
            {
                if (a.Name == "KKK")                  //tabpage名 kkk
                {
                    have = true;
                }
            }
            if (!have)
            {
                TabPage TabPageCity1 = new TabPage("KKK");
                TabPageCity1.Name = "KKK";
                this.tabControl1.TabPages.Add(TabPageCity1);
                FormCity frmCity = new FormCity();                           //FormCity：From
                frmCity.TopLevel = false;
                frmCity.Parent = TabPageCity1;
                frmCity.ControlBox = false;
                frmCity.Dock = System.Windows.Forms.DockStyle.Fill;
                frmCity.Show();
                this.tabControl1.SelectedTab = TabPageCity1;
            }
            else
            {
                MessageBox.Show("cz");
            }
            
        }

------解决方案--------------------

设置FormBorderStyle为None就没有标题栏了~