In this paper a two-stage physics-based model has been applied to study the evolution of plasma produced by high-intensity nanosecond laser ablation in vacuum under external magnetic field. In the early stage (Stage I), the laser-induced plasma generation and its short-term evolution are described through one-dimensional (1D) hydrodynamic equations. An equation of state (EOS) that can cover the density and temperature range in the whole physical domain has been applied to supplement the hydrodynamic equations. In the later stage (Stage II), the plasma long-term evolution is simulated by solving 2D gas dynamic equations. The two-stage model can predict the spatial distributions and temporal evolutions of plasma temperature, density, velocity, and other parameters. The model is used to study and discuss the effects of external magnetic field on the plasma evolution. It provides a useful tool for related fundamental studies and practical applications.