I am using watchdog timer which have adjustable window BUT ITS RESET delay is fixed.This reset pin is active high during normal operation and during fault condition its gives 200ms low pulse. So how can I drive power supply using this reset pulse?
As you already presumed, you can use a monostable multivibrator to extend your pulse, for example using a 555 timer. If you enter "555 timer monostable" this is the first image that pops up:
Just connect your watchdog timer output instead of R2 and the switch and connect a transistor (with base resistor) to drive the relay on the OUT pin.
Select R1 and C1 so t is 10 seconds according to the formula in the picture.