It is difficult to meet the design requirements of high-frequency surface acoustic wave (SAW) filters with low insertion loss and large bandwidth using conventional Rayleigh surface acoustic wave design. In this paper, a high-frequency broadband low insertion loss longitudinal leaky surface acoustic wave (LLSAW) filter based on multi-layer film structure has been designed. Firstly, the waveguide structure exciting the longitude leaky surface acoustic wave (LLSAW) is determined, and the 2D theoretical model of the device is established. Secondly, the influences of various electrode parameters on the spurious are studied. It is found that when the grating reflector of the series resonator is set to 6, the spurious in the passband can be suppressed as much as possible. Finally, the mirror T-type and mirror π-type filters are designed, and the performances are compared. The results show that the mirror T-type structure is more effective in suppressing the spurious response. The designed LLSAW filter with mirror T-type structure has a center frequency of 3 545 MHz, insertion loss of -1.416 dB, -3 dB bandwidth of 274 MHz, and out-of-band rejection of greater than -30 dB.