Abstract: Flange bolt loosening can be detected by using the flexible piezoelectric composite sensor. The effect of PZT particle size on the piezoelectric and dielectric properties of the composites was experimentally studied. The piezoelectric voltage constant g₃₃ of the prepared composites reached 117 mV·N^-1. The flexible composite material sensor was used to receive ultrasonic waves with different loosening degrees of single and multiple bolts, and the variation law of the first arrival wave amplitude of the ultrasonic transmission signal with the loosening of the bolts was obtained. The experimental results have shown that as the loosening degree of a single flange bolt increases, the amplitude of the first arrival wave of the ultrasonic transmission signal decreases. Under the same pre-tightening torque of flange bolts, the first arrival wave amplitude of ultrasonic transmission signal decreases with the increase of the number of loose bolts, which shall provide a theoretical basis for quantitative detection of flange bolt looseness.