Applications of the spectral analysis methods discovered by Kirchhoff for the study of stellar magnetic fields are considered. Statistical properties of mean magnetic fields of OBA-type stars were investigated basing on the information from two magnetic field catalogues. It is found that the mean effective magnetic field Ɓ can be used as a statistically significant characteristic of a stellar magnetic field. The magnetic field distribution function F(Ɓ) was derived for B-type stars. It turned to be a power law, a sharp de crease of F(Ɓ) was found in a weak magnetic fields range. Statistical properties of magnetic fluxes of Main-Sequence stars, neutron stars and white dwarfs are analysed.