  PSR B0540-69 has braking index measurement in its persistent state: n=2.129
\pm 0.012. Recently, it is reported to have spin-down state changes: a suddenly
36 % increase in the spin-down rate. Combining the persistent state braking
index measurement and different spin-down states, PSR B0540-69 is more powerful
than intermittent pulsars in constraining pulsar spin-down models. The pulsar
wind model is applied to explain the variable timing behavior of PSR B0540-69.
The persistent state braking index of PSR B0540-69 is the combined effect of
magnetic dipole radiation and particle wind. The particle density reflects the
magnetospheric activity in real-time and may be responsible for the changing
spin-down behavior. Corresponding to the 36 % increase in the spin-down rate of
PSR B0540-69, the relative increase in the particle density is 88 % in the
vacuum gap model. And the model calculated braking index in the new state is
n=1.79. Future braking index observation of PSR B0540-69 in the new spin-down
state will be very powerful in distinguishing between different pulsar
spin-down models and different particle acceleration models in the wind braking
scenario. The variable timing behavior of PSR J1846-0258 is also understandable
in the pulsar wind model.
