Balmer decrement defined by the ratio of spectral intensity defined by I(Hα) /I(Hβ) is applied tentatively to the recent outbursts of three symbiotic stars (AG Peg,V 694 Mon and AG Dra). As a result, during the outbursts,a characteristic feature can be recognized between their brightness change (from AAVSO data base) and the Balmer decrement obtained from our low-resolution spectroscopic observational data.The resulting feature seems to coincide partially with the formula derived from the simplified theory based on the Rayleigh scattering.