As critical transportation links connecting coastal urban agglomerations， the seismic design and safety assessment of underwater immersed tunnels are essential. Therefore， this study considered the Hong Kong-Zhuhai-Macao Bridge immersed tunnel （a two-hole and one-duct structure） as the research object. First， a two-dimensional finite element model of the tunnel-soil system was established using the OpenSees open-source numerical platform， the physical and mechanical parameters of soil layers were determined according to the site conditions， and three types of ground motions were selected as seismic inputs. Second， the relative deformation ratio， inter-story drift ratio， and transverse inclination angle of tunnel bores were chosen as damage measures （DMs） to characterize seismic performance； the corresponding ground motion intensity measures （IM） were determined based on a probabilistic seismic demand model. Finally， seismic fragility curves for the immersed tunnel were derived using an incremental dynamic analysis method. The results indicate the following.（1） The peak ground velocity is the optimal IM for the selected DMs.（2） According to the fragility curves， the damage probability for the relative deformation ratio is the highest， whereas those for the inter-story drift ratio and transverse inclination angle are relatively close.（3） The damage severity of the immersed tunnel follows the order of near-fault pulse-like ground motions ＞ near-fault nonpulselike ground motions ＞ far-field ground motions. The numerical modeling approach， analysis methods， and conclusions of this study can provide invaluable references for the seismic performance evaluation and risk analysis of immersed tunnels.