The stochastic stability of a two degrees-of-freedom system under combined harmonic and Gaussian white noise excitation were investigated. Firstly， the non-autonomous system was transformed into an autonomous system by increasing the dimension. Secondly， using both singular perturbation and double Fourier series， the approximate analytical solutions of the moment Lyapunov exponents and the largest Lyapunov exponents were obtained， which agree well with the results obtained by the Monte Carlo simulation. Finally， based on the moment Lyapunov exponents and the largest Lyapunov exponents， the effects of subharmonic resonance and combination additive resonance on the stochastic stability of the two degrees-of-freedom system were discussed.