HttpRunTime增加
requestValidationMode="2.0"
并在页面上设置

validateRequest="false" 
如果想只针对某些提交数据放行，可以这样写

public class RequestValidatorDisabled : System.Web.Util.RequestValidator
{
    protected override bool IsValidRequestString(System.Web.HttpContext context, string value, System.Web.Util.RequestValidationSource requestValidationSource, string collectionKey, out int validationFailureIndex)
    {
        if (value.IndexOf("<div>") >= 0)
        {
            validationFailureIndex = -1;
            return true;
        }
        else
        {
            return
            base.IsValidRequestString(context, value, requestValidationSource, collectionKey, out validationFailureIndex);
        }
    }
}

requestValidationType="System.RequestValidatorDisabled"