74121 monoshot ... is getting false triggered.
nothing unusual. I refer to them as "one shot in the foot". Nothing is more noise sensitive than a one-shot, AVOID IF AT ALL POSSIBLE.

So i want to use an optocoupler MCT2E at the input. So please tell me regarding the connections of the optocoupler..
Make what now drives your input drive the diode, make the transistor drive your input.

Go ahead and try the optocoupler, but if you stop shooting yourself in the foot, you will, most likely, have a greater chance of success.

Let us know why you are using the darn thing.

Erik