The 1-3 piezo-composites with high electromechanical coupling coefficient and low acoustic impedance are suitable for fabricating hydrophone and medical ultrasound transducers. However, the lateral mode resonances of 1-3 piezo-composites will reduce the performance of transducers and limit the miniaturization and high frequency of transducers. In this work, a set of 1-3 piezo-composites with rectangular ceramic pillars are designed. The thickness of piezo-composites is 0.32 mm. The width of ceramic pillars is 0.08 mm and the length of ceramic pillars varies from 0.10 mm to 0.25 mm, the kerf is 0.05 mm. The modeling and calculating has been carried out by using the PZFlex finite element software. The simulation results show that the thickness mode resonant frequency and anti-resonant frequency of the composites are 4.5 MHz and 6 MHz, respectively. The lateral mode resonant frequency gradually shifts to the low frequency with the increase of the length of the ceramic pillars. The thickness mode and lateral mode are coupled when the ratio of length to height of ceramic pillar is greater than 0.69. The transducer is designed based on the composite and the center frequency of transducer is 4.7 MHz according to the PiezoCAD software.