For the scattering of unpolarized natural light transfering in real atmosphere, the scattering parameters about which we are concerned are usually the intensity of scattered light and degree of linear polarization. In this paper, based upon the statistics about the scattering probability of photons along randomly sampled incident routes, a succinct numerical method has been developed to compute the phase function and degree of polarization for the single scattering of light by polydispersive large particles. The difficulty related to divergence, when the surface of scattering particle is curved, has been avoided by this numerical method. Meanwhile, because the shapes of water droplet in atmosphere are often spheroids approximatively, the scattering characteristics of spheroidal water droplets have also been discussed.