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Multi-photon ionization (MPI) and avalanche ionization (AI) have become main ionization mechanism of laser-induced optical dielectric film material damage, the different effects of MPI and AI for longer and shorter pulse are also discussed respectively. Based on Stuart et al.'s model equation and by means of numerical method, this paper has studied the interrelation of laser threshold intensity (Ith) versus laser pulse duration (τ) and damage threshold fluence (Fth) versus τ , and the effect of MPI and AI versus τ, studied electron density evolution in fused silica dielectric films induced by single-pulse laser. The results show that for fused silica film, for longer pulse duration τ, AI plays leading role and MPI serves for production of seed electrons for AI; for shorter duration τ, MPI predominates the prophase of the pulse duration. After stable MPI established, AI will exceed MPI and dominate the later period.