Why can't atduino nano read from apm telemetry port?

I understand the apm AC is sending out heartbeat msg from telemetry port
via Serial 0 in 57600 baudrate
But when connect an arduino uno or nano with apm,
Vcc gnd tx from apm to arduino 5v,gnd,rx0,
why can not get data from apm port?
Very simple code as following
setup ()
{
Serial.begin(57600);
}
Loop()
{
while(Serial.available()>0)
{
Int i;
I = Serial.read();
// it seems the nano never run to this point!!

}
I checked near everything,
even use a minimosd to connect the same
port 3 pins, the minimosd works well.

@Wilsonchang,
What does your issue have to do with QGroundControl? Please explain.
Regards,
TCIII AVD