Abstract: Abstract By analyzing the damage characteristics of composite hat-stiffened panels, piezoelectric-based guided wave monitoring technology was introduced into the damage monitoring of hat-stiffened panels. Finite element simulation was firstly conducted on the propagation mechanism of guided waves in hat shaped reinforcement structure. The simulation results showed that the propagation of guided waves in the hat-shaped reinforcement exhibited a complex characteristic of first separation and then convergence with the structural form. In order to quantify guided wave signals, two damage index algorithms with energy and correlation as measurement criteria were proposed. Finally, the piezoelectric-based guided wave technology with two damage index algorithms was applied to the fatigue testing of composite hat-stiffened panels. Experimental results showed that the propagation of lamination damage could cause significant signal changes of guided waves. The energy damage index and correlation damage index algorithms could identify the propagation of lamination damage on the hat-stiffened reinforcement. Although hat-stiffened reinforcement increased the complexity of guided wave propagation, piezoelectric-based guided wave technology was still effective for monitoring the damage of reinforcement bars in composite hat-stiffened panels.