About the solution EMC/EMI of AD421
What condition occur this symptoms in AD421?



You are probably violating the absolute maximum ratings of the part, since you don't have a schematic, I'll take a guess as to what is going on: There is an overvoltage to the part, and you didn't protect the rails. Or there is an undervoltage for a short duration. Or the inputs go to an external device or antenna and you didn't put voltage limiting diodes to protect the inputs and outputs of the AD421

Drop a schematic in that answer and I'll update mine


What is "POWER-ON RESET" in internal functional block diagram of AD421?



From the datasheet:


  "When REF OUT1 and REF OUT2 are used in application circuits, external
  4.7uF capacitors are required on the reference pins to provide compensation and ensure stable operation of the references. These
  capacitors can be omitted if the internal refer- ences are not
  required. REF OUT2 is sensed internally, and if more than 0.5 mA is
  drawn externally from this reference, the chip goes into a power on
  reset state. In this state the sigma-delta DAC is disabled, the
  internal oscillator is stopped and the input data latch is cleared"


So you could be drawing more than 0.5mA from the Refout2 port. and that is shutting down the part. The first thing I would do is check your Vcc when preforming the surge test with an oscilloscope. I would also check your ground voltage vs an external ground. 

Could you give me the anything reference material of AD421 for the solving EMC/EMI?


Yep, Its called Electromagnetic Compatibility Engineering