I try using the WCLogical.Or, but still return empty. If I don't add this filter Criteria, I can get the disired worklistitem, and the ProcessDataField value is correct.

If I replaced with below:

crit.AddFilterField(WCLogical.And,WCField.ProcessData,"DealerCode",WCCompare.NotEqual,dealerCode);

then will return all the Worklistitems which include the aimed WorklistItem. So it's very strange.