but why still need to add Max 232 module to make it communication is well.
Because RS232 signals are not the same as the TTL signals you need for the HC-05.Basically an RS232 has -12V for a mark and +12V for a space, where as TTL has +5V for a mark and 0V for a space. So you need a converter to change the voltages and invert the signal.
Part A ( Fixture side ) : DUT <-> ATEN (USB to TTL) <-> HC-05 = Communication is FAIL.
Is this a new situation, one that is not covered in the diagram you posted?Because a USB to TTL should be the same as a USB to RS232 + a RS232 to TTL
As I said it is probably some handshake line on the TTL that has not been asserted.