public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
        }

        private Form1 fm;
        public Form2(Form1 fm)
        {
            InitializeComponent();
            this.fm = fm;
        }
        private void button1_Click(object sender, EventArgs e)
        {
            fm.toolStrip1.Items[0].Text = "test";
        }
    }

------解决方案--------------------
把他封装成一个对象
用构造方法传过去