There are plenty of local copies of pages of remote web sites on local databases on most of web search engine sites. It is necessary to pull remote web pages periodically to refresh local copies of these pages on database in order to keep copies and source pages consistent，and which takes plenty of time and resources.The article proposes serveral policies to synchronize copy and source pages , proposes a Poission model of source page change, define freshness and fresh time, studies on these policies and compares their effectiveness.It is shown that the proposed policies improve the freshness of web pages significantly.