A new approach is proposed to approximate the stochastic salvo model with characteristic functions. By the distribution of the difference of the total number of well-targeted offensive missiles and the total number of well-targeted defensive missiles per salvo and the characteristic function of random sum of independent random variables, the characteristic function of the nominal surviving force strength is derived. Then the distribution function of the nominal surviving force strength is obtained by the inversion formulae, and the probabilities of some outcomes of one exchange of salvos can be calculated by the distribution functions. This approach can be used to analyze the stochastic salvo model precisely.