using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Threading;

namespace ParaThread
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            setTextDel = new SetTextBoxTextDelegate(this.SetText);
        }
        delegate void SetTextBoxTextDelegate(TextBox textbox,string text);
        SetTextBoxTextDelegate setTextDel = null;
        private void SetText(TextBox textbox, string text)
        {
            textbox.Text = text;
        }
        private void btnCountA_Click(object sender, EventArgs e)
        {
            ParameterizedThreadStart ptsA = new ParameterizedThreadStart(this.process);
            Thread thA = new Thread(ptsA);
            thA.IsBackground = true;
            thA.Start(txtNumA);
        }

        private void process(object textBox)
        {
            TextBox tb = (TextBox)textBox;
            for (int i = 0; i < 9999999; i++)
            {
                this.Invoke(setTextDel,tb,i.ToString());
                //Thread.Sleep(100);加上Sleep,两个文本框能同时计数
            }
        }

        private void btnNumB_Click(object sender, EventArgs e)
        {
            ParameterizedThreadStart ptsB = new ParameterizedThreadStart(this.process);
            Thread thB= new Thread(ptsB);
            thB.IsBackground = true;
            thB.Start(txtNumB);
        }
    }
}

WindowsFormsSynchronizationContext Sync = new WindowsFormsSynchronizationContext();
private void SetText(TextBox textbox, string text)
        {
            Sync.Send(_=> { textbox.Text = text; },null);
        }

------解决方案--------------------
你每次执行process方法的时候，每次循环都会更新一次UI，UI线程都会this.BeginInvoke（...）中...处的代码占用，而一次循环的操作间隔太短，因此阻塞是不可避免的。
你可以通过Thread.Sleep(1);来让当前循环休眠一下，这样UI就不会被阻塞了。

总之就是你process里面一次循环的间隔时间稍微长一点的话，UI的更新就不会频繁到阻塞了。