The physical mechanism of optical breakdown induced by short-pulse laser in water was analyzed, and the laser breakdown threshold was determined with a numerical calculating method for the rate equation of free electron density. The numerical calculating results were compared to experimentally measured breakdown threshold of pure water and impure water by nanosecond,picosecond and femtosecond laser pulses in the visible and near-infrared.On this basis, the evolution of the electron density and the absorption coefficient and energy density of the plasma during the laser pulse were gotten. A good quantitative agreement was found between calculated and measured values.