We established a simple mathematical model that the degree of polyamine stimulation of mRNA translation was dependent on the structure of the Shine-Dalgarno (SD) sequence in Escherichia coli. Loose base pairing of the SD sequence with other regions of mRNA caused strong polyamine stimulation, while intense base pairing of the SD sequence with other regions of the mRNA resulted in insignificant weak polyamine stimulation. The result suggested that the protein expression level was relative to the exposure probability of the SD sequence from its base paired state by the polyamine stimulation. The model agreed well with the experimental analysis.