The phenomenon of stochastic resonance (SR) in a bistable system driven by multiplicative and additive white noises and a periodic rectangular signal with a constant component is studied according to the theory of signal-to-noise ratio (SNR) in the adiabatic limit. The analytic expression of the SNR is obtained for arbitrary signal amplitude without being restricted to small amplitudes. We find that the effects of the multiplicative noise intensity D and additive noise intensity α on SNR are different: the SNR-α curve shows SR for almost the whole range of static asymmetry r, but the SNR-D curve only displays SR for some values of static asymmetry r. It is more sensitive to control SR through adjusting the additive noise intensity α than adjusting the multiplicative noise intensity D, when the asymmetry of bistable potential r is not too large. Moreover, the static asymmetry r decreases the SNR.