In this paper an improved 3IDT-LCR structure was developed by theoretical simulation, and a low-loss SAW filter with small rectangular coefficients was successfully developed based on this structure. The device adopts 42°Y-X LiTaO3 piezoelectric substrate with nominal frequency 253.75 MHz, measured insertion loss 1.05 dB, lower-side stopband rejection nearly 80 dB, higher-side stopband rejection nearly 65 dB, -1 dB bandwidth 10.52 MHz, -3 dB bandwidth 12.54 MHz, -40 dB bandwidth 16.68 MHz, and rectangular coefficient 1.33. The measured results show that the SAW filter with improved 3IDT-LCR structure can achieve the performance characteristics of low insertion loss, small rectangular coefficient, and high stopband rejection at the same time.