A new biaxial Bouc-Wen model for seismic dynamic analysis of an inelastic Two-Degree-of-Freedom (2DOF) system under bidirectional horizontal excitations was developed by adopting the normalized displacement as a governing parameter and taking P-A effect, strength deterioration, stiffness degradation and strain hardening into account. In this model, the circular yield surface was used to describe the coupling effect between orthogonal normalized restoring forces. The influences of bidirectional excitations and P-A effect on statistical characteristics of seismic ductility demand of the inelastic 2DOF system were quantitatively investigated using 69 earthquake records. Analysis results showed that bidirectional excitations and P-A effect can significantly affect seismic ductility demand of the inelastic 2DOF system depending on structural parameters; the seismic ductility demand of systems with long period can be described as either Lognormal or Frechet distribution variable, while Frechet distribution is preferred for systems with short period; the seismic ductility demand of the inelastic 2DOF system under bidirectional excitations can be described as a square root of the square sum of seismic ductility demands of single-degree-of-freedom (SDOF) systems under unidirectional excitation.%利用规一化位移作为控制参数,以圆形屈服面来描述双向规一化抗侧恢复力之间的耦合效应,且综合考虑P-Δ效应、强度退化、刚度退化、应变硬化等因素的影响,建立了双向地震作用下非弹性双自由度体系地震动力响应分析的新型双轴Bouc-Wen模型.结合69条地震记录,定量地分析了双向地震激励和p-Δ效应对延性需求的概率统计特征的影响.计算结果显示:双向地震激励和P-Δ效应对延性需求的影响较大,且影响程度对结构参数的取值比较敏感；长周期体系的延性需求可以描述为Lognormal或Frechet分布,而短周期体系的延性需求描述为Frechet分布更为合理；双向地震激励下的延性需求可以近似表达为单向地震作用下延性需求的平方和开平方.
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