A three-dimensional ultrasonic imaging method and a three-dimensional ultrasonic imaging system. The method comprises: transmitting an ultrasonic wave to a fetal head; receiving an ultrasonic echo, obtaining an ultrasonic echo signal, and obtaining the three-dimensional volume data of the fetal head according to the ultrasonic echo signal; according to the characteristics of a median sagittal section of the fetal head, detecting the median sagittal section in the three-dimensional volume data; and displaying the median sagittal section. The ultrasonic imaging method and the ultrasonic imaging system can obtain the three-dimensional volume data of the fetal head by ultrasonically scanning the fetus and can also automatically detect and display the median sagittal section of the fetal head, thus solving the problem that it is difficult for a doctor to precisely and manually position the median sagittal section.