Phase control method of piezoelectric transformer DC DC converter has the feature of higher efficiency and more stable output compared with pulse width modulation（PWM）,pulse frequency modulation（PFM） and PWM/PFM mixed control. A new phase discriminator is designed and measured. The relation curve between phase difference and frequency and relation curve between phase difference and duty are consistent respectively under different load, so the phase control algorithm is put forward to realize the resonance frequency tracking and to regulate output voltage of DC DC converter under narrowband (1 kHz) condition. MPT300650L0 and PT3906BR are taken as examples to illustrate the proposed method with experimental research. The experimental results show that DC DC converter phase control is feasible and versatile. When the input voltage is 3.3 V, the output voltage is 100 V.