In this study, the velocity change mode of a transversely coupled ultra-narrow band surface acousticwave filter is analyzed using the waveguide method.The changes in the central coupling strip structure and couplingdistance of the transversely coupled surface acoustic wave filter are modeled and calculated using the multi-physicsCOMSOL software, and the frequency response characteristics are analyzed.An ultra-narrow band SAW filter withfrequency of 898.144 MHz and insertion loss of 6 dB has been designed and verified.The results show that the fractionalbandwidth of surface acoustic wave filter using the center coupled bar with an open cavity reaches 7.2‰,which is 2‰ higher than that of the non-open cavity structure of 5.2‰.This indicates that the change of the centercoupled bar structure of a transversely coupled surface acoustic wave filter can change the fractional bandwidth of ultra-narrow band surface acoustic wave filters.