Abstract:  The present paper aims to research the acoustic fields within and outside baiji’s (Lipotes vexillifer) head through finite element method. The harmonic point sound source with a frequency range of 50 kHz to 140 kHz was used to reveal the coincidence between frequency and far-field acoustic parameters, main beam angle and -3dB width. Different from previous studies which neglected the shear wave of solid skull structures (mainly maxilla and cranium) and regarded them as fluid medium, the present work takes the shear waves of solid structures into consideration. Two sets of models, one of which allowed for the shear waves to engender in the solid skull structures and the other not, were compared to investigate what effects would be caused by the presence of shear waves. Three special frequencies, namely 70 kHz, 90 kHz and 110 kHz, were specifically chosen to obtain their sound wave propagation processes within the head. The results suggested that in the models which considered the shear waves in the solid structures, its presence mainly affected the near-field acoustic fields and merely caused slight differences in main beam angle and -3dB band width, compared to those of models which ignored the shear waves of skull structures. The reflection waves were found to change with frequency. Solid interfacial waves were observed inside the maxilla of the models considering the shear waves and its propagation patterns were also related to frequency. The solid displacements inside the maxilla indicated that the wave propagation patterns was changing with the thickness of the maxilla. Besides, in some special frequency, the sound wave scattered in the region among nasal passage, premaxillary sac and vestibular sac, resulting in weakening the sound energy propagating forward. The maxilla was thought here to play anther role as an interfacial wave guider besides its commonly accepted reflection role in beam formation. The study further indicates the biosonar emission system of baiji is complicated.