Question about issue connecting GPRS shield to adruino2560

Why after I power on the GPRS shield that plugged in my adruino, the IDE will disconnect my port(COM3)?

what type of GPRS shield are you using? There are many.

If it is the GSM-shield with Quectel M10 then please read "getting started" for that shield on the Arduino Products page. You have to modify one pin for the Mega2560.

I had similar problems when I tried to use RX0, TX0 for communication with another board. RX0, TX0 is reserved for communication with the IDE. This is valid for the Uno, the Mega2560 and many other boards.