1. Should the master using the Loop to montior and intake the serial port data?
or using serial interrupt to check the incoming data?
Serial = interrupt | trouble

2. I may using the array or string to store the incoming data.
Which method is easily and better?
either, however the usual is a circular buffer (do not attempt to process the data in the ISR)

3. there may be some missing byte during sending data so that may lead to system hang.
How can i avoide it?
timeouts
4. As i think the Packet length seems fixed. Is it
better to reduce the Length byte in the Packet? It may be an idea to keep the length byte - especially if you may have to upgrade the master and create another slave in the future - that way if you ever need to go to variable length there is no major changes.

5. Any problem in my protocol?
none, but what may be related to what you do which I do not know what is.

Erik