Code
using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

using System.Collections.Generic;
using System.Text;
using System.IO;
using System.Text.RegularExpressions;
using System.Data.SqlClient;
using Lucene.Net.Analysis;
using Lucene.Net.Documents;
using Lucene.Net.Index;
using Lucene.Net.Analysis.KTDictSeg;
using LuceneSearch;

/// <summary>
/// CreateIndex 的摘要说明
/// </summary>
public class CreateIndex
{
    //词库路径
    public string wordPath;
    public string indexDirectory;
    //定义一个IndexWriter
    protected IndexWriter writer = null;
    //需要导出的数目
    public int allNum;
    //当前完成的数目
    public int completeNum;
    //需要生成的表
    public DataTable dt;

DAL.OperSql os = new DAL.OperSql();
    public CreateIndex()
    {

}

public void GetIndex(int inum)
    {
        //定义分析器
        Analyzer KTDAnalyzer = new KTDictSegAnalyzer(wordPath);

//PerFieldAnalyzerWrapper可以对不同的Field进行不同的分析
        PerFieldAnalyzerWrapper wrapper = new PerFieldAnalyzerWrapper(KTDAnalyzer);

wrapper.AddAnalyzer("ID", KTDAnalyzer);
        wrapper.AddAnalyzer("News_Url", KTDAnalyzer);
        wrapper.AddAnalyzer("News_Date", KTDAnalyzer);

//判断是否已有索引
        bool isure = !IndexReader.IndexExists(indexDirectory);
        //创建索引的数据条数
        allNum = dt.Rows.Count;
        //创建IndexWriter
        writer = new IndexWriter(indexDirectory, wrapper, isure); 
        writer.SetUseCompoundFile(true); //显式设置索引为复合索引
        writer.SetMaxFieldLength(int.MaxValue); //设置域最大长度为最大值
        writer.SetMergeFactor(allNum + 100); //设置每100个段合并成一个大段
        writer.SetMaxMergeDocs(10000); //设置一个段的最大文档数
        writer.SetMaxBufferedDocs(1000); //设置在把索引写入磁盘前内存里文档的缓存个数
        //创建IndexReader
        IndexReader reader = null;
        bool needre = inum == 1;

reader = IndexReader.Open(indexDirectory);
        for (int i = 0; i < dt.Rows.Count; i++)
        {
            completeNum = i + 1;
            string body = parseHtml(dt.Rows[i]["News_Body"].ToString());
            string title = parseHtml(dt.Rows[i]["News_Title"].ToString());

if (title.Length > 2 && body.Length > 2)
            {
                if (needre)
                {
                    Term term = new Term("ID", dt.Rows[i]["ID"].ToString());
                    reader.DeleteDocuments(term);

}
                Document document = new Document();

document.Add(new Field("ID", dt.Rows[i]["ID"].ToString() ?? "", Field.Store.YES, Field.Index.UN_TOKENIZED));
                document.Add(new Field("News_Title", title, Field.Store.NO, Field.Index.TOKENIZED));
                document.Add(new Field("News_Body", body, Field.Store.NO, Field.Index.TOKENIZED));
                document.Add(new Field("News_Url", dt.Rows[i]["News_Url"].ToString() ?? "", Field.Store.YES, Field.Index.UN_TOKENIZED));
                document.Add(new Field("News_Date", DateField.DateToString(Convert.ToDateTime(dt.Rows[i]["News_Date"].ToString())) ?? "", Field.Store.YES, Field.Index.UN_TOKENIZED));
                writer.AddDocument(document);;
            }
        }
        reader.Close();
        writer.Optimize();
        writer.Close();

}
}

Code
try
        { 
            ci.wordPath = Server.MapPath("App_Data") + @"\"; //词库路径;
            ci.indexDirectory = Server.MapPath("index") + @"\"; //词库路径;
            ci.dt = dt;
            ci.GetIndex(1);           
        }