In this study, a torsion bi-stable mechanism is proposed to enhance the absorption efficiency of the raft-type wave energy converter (WEC). The torsion bi-stable mechanism and a power take-off (PTO) system are settled at the joint between hinged rafts. The dynamic model in the time domain of the raft-type WEC has been established in conjunction with a simplified model of the bi-stable mechanism. In order to analyze the performance of the whole system more rapidly, the convolution integral term of the dynamic model has been replaced by the state space model. The performances of the WECs are analyzed in both regular and irregular sea conditions. Compared with the corresponding linear model, the numerical results indicate that the bi-stable mechanism can enhance the capture efficiency of the raft-type WEC significantly and extend the range of the high efficiency to the lower frequencies.