In this study, a high rectangular and low loss trapezoidal SAW (Surface Acoustic Wave) filter based on Rayleigh wave mode is designed to ensure the spectrum isolation of the mid-frequency band between different IoT devices in the 5G era. The electric electrode of the filter is the Al electrode, and the piezoelectric material is 128° YX-LiNbO₃. To realize the design requirement of low loss, the influence of electrode thickness, duty cycle, aperture, and reflector gate on the resonator performance is simulated by the finite element method. Simultaneously, a non-standard short-circuit gate structure is adopted to suppress the spurious in the passband. A new image topology is designed to improve the waviness on both sides of the passband of general topology filters. Furthermore, the design goal of high rectangularity is achieved by reducing the capacitance ratio of the filter (C_0p/C_0s). Finally, a six-step SAW filter with a center frequency of 315.03 MHz, -3 dB bandwidth of 11.62 MHz, minimum insertion loss of 0.758 dB, out-of-band rejection of 35.931 dB, and rectangular coefficient of 1.30 is designed.