hi,
"cut-off time is the time that required MOSFET from on state to off state,is it right?How to calculate the cut-off time of a MOSFET"
"what is the difference between maximum output power and total power dissipation".

 

1) I'd run straight to the datasheet. Here's one!

On page 2, I see turn off delay time as 39 nanoseconds and fall time as 20 nanoseconds.
Of course, these are dependent on the conditions described in the datasheet and your ability to shut the gate off faster than the inherent time the mosfet requires.

If I missed your point, you are allowed to ask again.

Maximum Power Dissipation is the amount of power the chip can convert to heat without self-destruct from over heating. In this case, 125 degrees C above the assumed ambient of 25 C, and using a theoretically perfect (infinite) heat sink.

Maximum power output has a lot to do with the voltage and current applied to the load. This part is not available on the datasheet.