添加引用ICSharpCode.SharpZipLib.dll、NPOI.dll、NPOI.OOXML.dll、NPOI.OpenXml4Net.dll、NPOI.OpenXmlFormats.dll，这几个dll文件NPOI官网上只能下载到源码，编译有些麻烦。这里找到一个现成的，http://www.itmop.com/downinfo/267826.html。
源码如下。

using System;
using System.Collections.Generic;
using System.Windows.Forms;
using NPOI.XWPF.UserModel;
using System.IO;

namespace WindowsFormsApp1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            openFileDialog1.Title = "选择word文件";
            openFileDialog1.Filter = "Word文档|*.docx";
            if (openFileDialog1.ShowDialog() == DialogResult.OK)
            {
                textBox1.Text = openFileDialog1.FileName;
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            //遍历word文档所有段落，并存入文本文件
            FileStream fs = new FileStream(textBox1.Text, FileMode.Open);
            XWPFDocument doc = new XWPFDocument(fs);
            IList<XWPFParagraph> paragraphs = doc.Paragraphs;
            foreach(XWPFParagraph paragraph in paragraphs)
            {
                File.AppendAllText("⽂本1.txt", paragraph.Text);
            }
            
        }
    }
}