The stochastic resonance (SR) in an optical bistable system under the simultaneous action of multiplicative and additive noises and periodic signal is studied using the adiabatic approximation theory. The signal-to-noise ratio(SNR)is a non-monotonic function of intensity of additive noise, in which the SR phenomenon appears. The SNR decreased with the intensity of multiplicative noise, and SR phenomenon does not appear. So the effects of additive and multiplicative noise on the output SNR are different.