尝试在下拉列表中将autopostback设置为true

Try setting autopostback to true in your dropdownlist

public void Button2_Click(object sender, EventArgs e)
   {
       try
       {


           SqlDataAdapter da = new SqlDataAdapter("select pcode,fyyear,date,salary,ta,contigency,nrc,institcharges,others from monthly where pcode=(''" + DropDownList1.SelectedItem.ToString() + "'' ) AND fyyear=(''" + DropDownList2.SelectedItem.ToString() + "'')", con);
           DataTable dt = new DataTable();
           da.Fill(dt);
           GridView1.DataSource = dt;
           GridView1.DataBind();

           DropDownList1.SelectedIndex = 0;
           DropDownList2.SelectedIndex = 0;
       }

       catch (Exception ex)
       {
           ex.Message.ToString();
       }


   }