I execute the follow statements:

ISession session = Sessions.GetSession();
ICriteria crit = session.CreateCriteria(typeof(PubInfo));
crit.Add(Expression.Like("PrInfo", "%"));
crit.AddOrder(Order.Asc("PubId"));
IList list = crit.List();

DataGrid1.DataSource = list;
DataGrid1.DataBind();

session.Flush(); // here,NHB generate 'Update xxx set xxxx' for every row in IList, Why?

Depends on what the xxx actually is. Probably there was some property that has changed, maybe as a result of loading (like NULL changing to 0 or something like that).

the xxx is 'update PubInfo set column_1=@p1....column_n=@pn where ...'

'NULL changing to 0', This is the NHB internal processing, how solves this problem?

If this is really your problem, then you can either use nullable types (from Nullables in NHibernateContrib package) or write your own usertype that will transform NULL to 0. But first you have to make sure this actually is your problem.