Abstract: When the gas of the chemistry-pure polyatomic molecular is in a closed container and is dissociated into photofragments by the irradiation of the light of the special frequency, we can imagine easily that the pressure in the closed container will change. In this paper, from statistical principle, we have carefully deduced the law of the change between pressure and time,which will play a positive role in exploring the interaction law of light and matter.

Key words: polyatomic molecular, photodissociation, pressure