是不是用的AspNetSqlMembershipProvider?
检查下你的配置
<membership>
<providers>
<add name="AspNetSqlMembershipProvider"
type="System.Web.Security.SqlMembershipProvider"
connectionStringName="LocalSqlServer"
enablePasswordRetrieval="false"
enablePasswordReset="true"
requiresQuestionAndAnswer="true"
applicationName="/"
requiresUniqueEmail="false"
passwordFormat="Hashed"
maxInvalidPasswordAttempts="5"
minRequiredPasswordLength="4"
minRequiredNonalphanumericCharacters="0"
passwordAttemptWindow="10"
passwordStrengthRegularExpression=""
slidingExpiration="false" />
</providers>
</membership>
slidingExpiration如果设置成"true"的话,每次访问过期时间将会重置