We report that the measurements of the pyroelectric current of the pre-poled [111]-oriented 0. 955 Pb(Zn1/3Nb2/3)O3-0.045 PbTiO3 (PZN-4.5%PT) single crystals can shed some light on the phase transition and spontaneous polarization characters of this material in a similar way to measures of remanent polarization and dielectric properties. The pyroelectric current is measured and the corresponding spontaneous polarization is calculated as a function of temperature with various poling fields added during cooling the sample from 200℃ to room temperature. Critical electric field of 0.061 k V/cm is found to be essential to induce the intermediate ferroelectric orthorhombic phase between the ferroelectric rhombohedral and tetragonal phases. Below the critical field, the polarization increases almost linearly with the increase of poling field. At the critical field, the polarization at 30℃ increases abruptly from 14μC/cm2 for a poling field of 0.06kV/cm to 29.5μC/cm2 for a poling field of 0.061 kV/cm, and afterwards, increases slowly and saturates to 31 μC/cm2 for poling fields beyond 0.55 kV/cm.