Hi @Eric_Wang,
I am unfamiliar with the GPRS library that you are using, but what does gprsAccess.attachGPRS() return? Reading through the example that Arduino provides, it looks like this function establishing a connection.
To check to see if the your GPRS access is still available, I would recommend that you use something like the available() method that this library provides. The example that I linked in the above paragraph has an example on how this is used, as well as a reference here.
Lots of things can cause this looping to occur, though. Here is a great post on some of the debugging steps that you can take if the above code changes do not help.
Let me know if this doesn’t help and you need more guidance.
Thank you,
Heath