AT-cut quartz is employed as the piezoelectric substrate in the design of surface acoustic wave （SAW）filters to meet the demand for extremely narrowband SAW filters used in communication and electronic warfare systems. A longitudinally coupled dual-mode （DMS） filter topology is adopted to achieve the required relative bandwidth of less than 1‰. A study is then conducted on the suppression of spurious transverse modes that appear in the passband and high-end transition band of AT-cut quartz-based narrowband SAW filters. Finite element method（FEM） simulations are used to suppress these transverse modes by introducing a piston structure at the thickened ends of the electrode fingers. Finally, SAW filters are fabricated both with and without the piston structure, and their insertion losses are measured. The results show that the minimum insertion loss, relative bandwidth, and in-band ripple of the SAW filter are significantly improved when using the piston structure.