The mechanism of ultrashort laser interacting with biological tissues was investigated in detail and a model has been developed to calculate irradiance thresholds for laser induced optical breakdown in biological soft tissues. Calculation results show that for nanosecond or subnanosecond laser pulses, initial heating electrons in strongly absorbing media contribute significantly to avalanche ionization and optical breakdown threshold decreases with increasing tissue absorbance; for subpicosecond laser pulses, multiphoton ionization is the main mechanism for optical breakdown which is insensitive to linear absorption.