lxml里面Element对象有个find的方法，先找到对应的path，然后修改就好了

# -*- coding: utf-8 -*- 

from lxml import etree

xml_string = u"""
<?xml version="1.0" encoding="UTF-8"?>
<urlset>
<url>
<lastmod>2014-08-26</lastmod>
<changefreq>always</changefreq>
<priority>1.0</priority>
<data>
<display>
<name>zhongguo</name>


<address>zhongguo</address>


<search>http://www.sohu.com</search>
</display>
</data>
</url>
<url>
<lastmod>2014-08-26</lastmod>
<changefreq>always</changefreq>
<priority>1.0</priority>
<data>
<display>
<name>中国</name>


<address>中国</address>


<search>http://www.abc.com</search>
</display>
</data>
</url>
</urlset>
"""

root = etree.fromstring(xml_string)
for url in root:
    url_tag = url.find("data/display/search")
    if url_tag.text == "http://www.sohu.com":
        url_tag.text = "http://www.sina.com"
print etree.tostring(root)