I think I have got a wrong way of thinking, so I try again.
As oxalic acid is a weak acid, so it ionize slightly to give hydrogen ion. Therefore, adding dilute sulphric acid can provide an acidic medium for MnO4- to react with ethanedioate ion.
I think it should be right. But let me ask a further question.
Why do we need to acidify the some oxidizing agent, e.g. MnO4-? Will MnO4- without acidifiying have oxidizing property??