Ultrasonic atomization technology is widely utilized for its superior performance, characterized by small droplet size and controllable atomization rate. In order to meet the high-viscosity fluids atomization requirements in fields of pulmonary drug delivery, fuel cell and surface coating, the flow tube internal cavitation atomizer (FTICA) driven by two-dimensional vibration has been proposed with the atomization viscosity increased by 6 times compared with the mesh atomizer. However, the atomization mechanism governing FTICAs remain to be elucidated. In the study, the atomization mechanism of FTICA is therefore investigated, revealing that longitudinal vibration constitutes the critical determinant governing the atomization process. First, an atomizer with an eccentric structure was designed to adjust the amplitudes of the longitudinal and transverse vibrations of the microtube. Cavitation threshold analysis is presented, and in conjunction with vibration measurement data, longitudinal vibration is demonstrated to serve as the determinant governing both cavitation and atomization. Subsequently, the simulation of the vibration mode and harmonic response analyses is implemented on the FTICA under various eccentricity parameters, which validates the elliptical motion of the microtube and the influence of eccentric distance on vibration amplitude. Some experiments are implemented on the atomizers with different eccentric distances to measure the vibration amplitude and the corresponding atomization rates, which can be confirmed that the atomization rate is positively correlated with the longitudinal vibration amplitude.