Abstract: The expression of scattering power spectrum of radio waves by a quasi-equilibrium plasma in the presence of a magnetic field had been previously obtained by a number of authors with respective methods in investigating the incoherent scatter theory.Their results,based on the Boltzmann equation,are identical.In this paper,using the dressed test particle approach given by Rosenbluth and Rostoker (1962),but employing a diffrent mathematical thechnique from theirs,the same result is obtained.We still start from the Vlasov equation and the Poisson equation,but adopt the way of solving differential equation and repeatly taking Fourier transform.The key lies in applying the convolution theorem to relate the time autocorrelation of electron density fluctuations with the function (这里有图片19820407-312-1.GIF) which can be directly calculated from the unperturbed orbits of charged particles.Hence the procedure is comparatively concise and straightforward.