Abstract The oscillatory flow fields in standing-wave thermoacoustic devices with and without adding stacks are visualized by Particle Image Velocimetry (PIV) measurement. First, the fundamental frequencies for two resonators with different lengths are tested and the fitting curves of pressure and velocity amplitudes along the resonator are obtained. Then, the oscillatory flow fields at the end of the stacks are analyzed. Finally, the ejection vortices at the end of the stacks in two resonators with different lengths are exhibited. The results indicate that the ejection vortices at the end of the stacks in two resonators with different lengths are similar at same driving powers. It is found that the two factors of the resonator length and the relative position of the stacks in the resonator, respectively, influence the fundamental frequency and the velocity amplitude; these two factors determine the peak-to-peak displacement amplitudes of the working gas. In addition, it is shown that increasing the resonator length and the relative position of the stacks in the resonator, respectively, decrease the fundamental frequency and increase the velocity amplitude, can enlarge the ejection vortices, thus benefit the enhancement of the heat transfer between the fluid and the heat exchanger in the thermoacoustic devices.