Based on the relationship between ionizing damage effect and displacement damage effect under radiation degradation mechanism of pn-junction diode, and combined with the noise theory of radiation degradation of pn-junction diode, a change law of low frequency noise of pn-junction diode under radiation found. The inconsistency between the change laws of of two kinds of effects is found. Based on the experimental result, the relationship between two kinds of effects is judged. This relationship can explain the experimental result. This is very important for the device hardening research.