Novel method of piezoelectric resonance spectroscopy is introduced for nonlinear-optical crystals equivalent temperature measurement during frequency conversion of laser radiation. Equivalent temperature of the crystal heated by laser radiation is directly determined from its strongly temperature sensitive piezoelectric resonance frequencies. Method was applied for PPLN crystal temperature measurement during second harmonic generation of CW ytterbium fiber laser radiation. Temperature tuning curves of PPLN and its phase matching temperature dependence on pump power were precisely measured using concept of the equivalent temperature. Hysteresis of PPLN temperature and optical bistability of second harmonic power in respect to pump power were observed.