The equation for the two-particles cosmic-ray distribution function is derived by means of the Boltzmann kinetic equation averaging. This equation is valid for arbitrary ratio of regular and random parts of the magnetic field. For small energy particles the guiding-center approximation is used. On the basis of the derived equation the dependence between power spectra of cosmic-ray intensity and random magnetic field is obtained. If power spectra are degree functions for high energy particles (≈ 10 GeV nucleon-1), then the spectral exponent υ of magnetic field lies between ϱ and ϱ-2, where ϱ is the spectral exponent of cosmic-ray power spectra. The experimental data concerning moderate energy particles are in accordance with υ=ϱ, which demonstrates that the magnetic fluctuations are isotropic or cosmic-ray space gradient is small near the Earth orbit.