  We study entanglement negativity for evaporating black hole based on the
holographic model with defect brane. We introduce a defect extremal surface
formula for entanglement negativity. Based on partial reduction, we show the
equivalence between defect extremal surface formula and island formula for
entanglement negativity in AdS$_3$/BCFT$_2$. Extending the study to the model
of eternal black hole plus CFT bath, we find that black hole-black hole
negativity decreases until vanishing, left black hole-left radiation negativity
is always a constant, radiation-radiation negativity increases and then
saturates at a time later than Page time. In all the time dependent cases,
defect extremal surface formula agrees with island formula.
