Hi Yuan,
When displaying an order at the Order View, transaction /SAPAPO/RRP2, you can get the OrderID with the following steps:
1) Open the debugger by using ok-code /h and pressing enter twice.
2) Display the following variable: GT_ORDERS[1]-ORDERID
I described this with screenshots in SCN Document How to read order details from the liveCache with FM /SAPAPO/OM_ORDER_GET_DATA.
Regards,
Tiago