There is a power loss detection ,it detect the VDD_SRC,and
what the funtion of this ?If we can delete it? as the picture
Its function has been listed as you can see: to detect a loss or unacceptable droop on the VCC_SRC. And will trigger power-off sequence if that happens. Suggest to keep it.
If we can increase the voltage drop range ？
No.
if no this part the system will reset？
What do you mean? That is to detect voltage droop and then assert vin_pwr_bad to reset system.