The invention discloses a constant-pressure control system for output pressure of a piezoelectric pump. The system comprises a personal computer (PC), an NI acquisition card, an acquisition card junction box, a piezoelectric vibrator driving power supply, the piezoelectric pump, a measuring cylinder at an inlet of the piezoelectric pump, a No.1 pressure sensor at the inlet of the piezoelectric pump, a measuring cylinder at an outlet of the piezoelectric pump and a No.2 pressure sensor at the outlet of the piezoelectric pump. The invention further discloses a control method of the constant-pressure control system for the output pressure of the piezoelectric pump. Expected given pressure P'out at the outlet of the piezoelectric pump is input; after a piezoelectric control map figure query module acquires the expected given pressure P'out, pressure Pin at the inlet of the piezoelectric pump in the current state is read in real time; a piezoelectric control map figure query unit queries voltage amplitude V'in and frequency H'in of driving voltage required by the piezoelectric pump when control map figure output reaches the expected pressure P'out at the outlet of the piezoelectric pump; the piezoelectric vibrator driving power supply amplifies a voltage signal, and amplitude Vin and frequency Hin of driving voltage of the piezoelectric pump are obtained; and the value of pressure at the outlet of the piezoelectric pump is detected in real time and is fed back to the piezoelectric control map figure query unit.