I want to be able to align items precisely(x,y coordinate ), how could I do?
When I change an item's animate time, will happen a small position jumps. If there is the feature can deal with this problem?
You can use the guidelines/ rulers on your canvas to be able to align your elements. If you drag from the top ruler or side ruler, you can bring the guideline onto your canvas.
In addition to the information Hayley Provided, here some threads discussing image alignment: