I assigned Resource one 1 days to one activity,why the planned duaration is 3 days?

Hi,

On your print screen, you don't dispayed the expected activity.
(the one with SATA in the name is expected)

But it gives me enough information to understand.
Your first activity, on 2013-2-27 is finished.
Your second activity, on 2023-2-28 is also finished
The "%SATA%" activity is started : apparently, 1/2 day is real work, on 2013-2-27, so you should have 1/2 day left.
Resource should have no availability left, so planning finished "%SATA%" activity on 2013-3-1 (as soon as possible).
This activity starts on 2013-2-27 (real work) and ends on 2013-3-1 (planned left work) : duration = 3 days !!!
What is not pointed out, is that there is no work on this task on 2013-2-28. You could see this is respource planning or work plan reports, on real work allocation for the resource...

Regards.

it means that the due date change day by day

Not the due date, but the planned date. Yes, it is the real project life !
And the target of Project'Or RIA is to reflect as near as possible the real situation, to let the project leader take the good decisions, and be able reach his target.

A project leader should not be confused when planned dates don't respect target dates. It is real life.
It is his duty to adapt project constraint to put project on the rails and get back to the target.