An analytical expression of optical breakdown threshold for water has been abtained after simplifying the rate equation of free electron experimentally measured breakdown threshold picosecond and femtosecond laser pulses in the density.The calculating results have been compared with of pure water and impure water induced by nanosecond, visible and near-infrared. For nanosecond laser pulses, the generation of free electrons in distilled water is initiated by multiphoton ionization but then dominated by avalanche ionization. For shorter laser pulses, multiphoton ionization becomes more important, and the effect of collision and recombination on breakdown threshold losses gradually during breakdown. A good quantitative agreement is achieved between calculated and measured results.