Abstract: Rickett’s big­footed bat，Myotis ricketti can emit ultrasonic through the mouth to navigate and detect special objects in the process of echolocation，meanwhile，it changes various sizes of mouth opening angle constantly in a reasonable range.The purpose of this work is to study the effect of different mouth opening angle on emission sound field.Firstly，a series of cross­section images of Rickett’s big­footed bat head was obtained by scanning its head using X­ray microcomputer tomography(micro­CT)．However，masses of glitches and noises of images caused a thorny problem that these cross­section images were not to be used to build three­dimensional digital model directly.In order to eliminate plenty of glitches and noises，cross­section images were processed by an isotropic Gaussian smoothing filter and binarization processing.After modified initial cross­section images，an original three­dimensional digital model of head sample was constructed by using a three­dimension cone­beam volumetric reconstruction method，and other digital models with different mouth opening angel were structured by digital deformation within a reasonable range based on primary angel.These kinds of three­dimensional digital representations were composed of cube voxels，and different digital models required to be manually handled by reducing or increasing the number of cube voxels in the position of the mouth.Next，finite­difference time­domain method and Kirchhoff integral were used as numerical methods to calculate the acoustic field which including near­field and far­field.Finally，acoustic near­field pressure magnitude and radiation pattern were presented as results to reveal the relationship between mouth opening angle and emission sound field.The study shows that Rickett’s big­footed bat can control and adjust the directivity of radiation sound field by changing the size of mouth opening angle.Given an opening angle，the higher the frequency of the radiation sound wave is，the stronger the directivity of radiation beam is.At a given frequency，the directivity of radiation beam can become more clearly with the increase of the mouth opening angle.Depending on the adjustment of mouth opening angle，Rickett’s big­foot bat is able to possess the capacity for changing shapes of radiation pattern，and this kind of capacity can meet its unique needs by enhancing the accuracy of echolocation during flying and preying.