The magnetization reversal resulting from the thermal fluctuation is irreversible for overcoming the energy barrier, ant it is called the thermally assisted tunneling. In this paper the relaxation in magnetizaition reversal resulting from the thermal fluctuation is observed in Pr-Fe-B permanent magnet. The dependence of magnetic moment on the time natural logarithm is the same as that on the energy barrier in the thermally assisted tunneling. So the relaxation in magnetization reversal originates from the macroeffect of magnons which follow Bose distribution law. The critical size in the irreversible magnetization reversal obtained by the fluctuation field is on a nanometer scale and close to the theoretical domain wall size, indicating that the thermally assisted magnetization reversal undergoes the nucleation and de-pinning of domain wall. The increase of coupling volume will reduce the possibility of magnons tunneling in magnetization reversal due to the weakening effect of thermal fluctuation. The variation of fluctuation field with the field verifies the effect of exchange coupling in Pr-Fe-B magnets, and the calculated value of fluctuation field is consistent with the aftereffect of thermal activation. With the increase of temperature the thermal fluctuation energy increases, and though the aftereffect of thermal fluctuation weakens due to the exchange coupling, the ratio of thermal fluctuation aftereffect to coercivity increases in Pr-Fe-B magnet.