A model of describing optical discharge effect of double cladding fiber is proposed. The simulation results show that the velocity of optical discharge of double cladding fiber nonlinearly increases with the increase of power density in the fiber core and the velocity slowly increases at a higher power density. The threshold of optical discharge depends on the initial heated temperature. When the initial temperature is fixed, the threshold decreases with the increase of the diameter of fiber core. The thresholds of double cladding fiber with different core diameters are compared expenmentally, which are consistent with the model. Moreover, the comprison of different inner cladding diameters with the same core diameter shows that the threshold of larger inner cladding fiber is relatively high while the relationship between propagation velocity and power density is the same, which indicates that the core diameter is the main factor affecting optical discharge velocity.