According to the Langevin equation that describes the Brownian particle motion, the instantaneous power and average power of Brownian particle provided by periodic external force are analyzed. The mechanism of energy input to a bistable system to generate stochastic resonance is revealed. The theoretical analysis and numerical simulation manifest that the instantaneous power absorbed by Brownian particle changes periodically. The change frequency is twice that of periodical external force, and its amplitude and average are controlled by the noise intensity. The energy input form of periodic external force can be controlled effectively by changing the parameters of thermal environment where Brownian particle is located.