The scalability is an important metric which reflects theperformance changing property of the parallel system when the machine size and the problem size are scaled. In this paper, we first analyze the characteristics of three existing scalability models for parallel systems, i.e., the iso-speed model, iso-efficiency model and iso-ratio-of-parallel-overhead-to-computation model, prove the equivalence of the three conditions on which each model is defined, i.e., the isospeed condition, the isoefficiency condition and the iso-ratio-of-parallel-overhead-to-computation condition, and point out the limitations of these models when used to study the scalability of parallel systems. We then propose a new scalability model which reflects the scalability of parallel systems more directly and precisely. We prove the relationship of our model with the three formerly mentioned models, and applied our models to the investigation of issuses on scalability with three types of parallel systems characterized respectively by the Amdahl speedup model, the Gustafson speedup model, and a more realistic speedup model given by us. Case studies indicate that using our model, it is more efficient to solve the following problems: (1)investigating the scalability quantitatively; (2)comprehensively reflecting the effects of factors from program, machine and runtime environment on scalability; (3)providing suggestions on how to keep and improve the scalability of parallel systems.%文中首先分析了等效率、等速度和等并行开销计算比三种并行系统可扩展性模型的特点，论证了等效率、等速度和等并行开销计算比三种条件的等价性，并指出这三种模型在描述可扩展性时的不直观及其局限性.然后提出了一种新的可扩展性模型.此模型直观地反映出并行系统在机器规模和问题规模扩展时，其性能的扩展特性.实例研究表明，该模型能更有效地解决下列问题：(1)定量研究并行系统的可扩展性;(2)全面地反映程序、机器、环境方面的因素对可扩展性的影响；（3）指导如何保持和改进并行系统的可扩展性.
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