您没有将其指定为 null，您正试图插入一个空字符串 ('').您需要:

You have not given it as null, you're trying to insert an empty string (''). You need:

INSERT INTO [ABC] ([code],[updatedate],[flag],[Mfdate]) 
VALUES ('203', '6/12/2013','N/A', NULL) 

尽管实际上，如果您要插入日期，***以 YYYYMMDD 格式插入它们，例如:

Although really, if you're going to be inserting dates, best to insert them in YYYYMMDD format, as:

INSERT INTO [ABC] ([code],[updatedate],[flag],[Mfdate]) 
VALUES ('203', '20130612','N/A', NULL)