We model the evolution of massive magnetic stars in the Galaxy using our own population synthesis code. A comparison of our model magnetic field distribution with that obtained from an analysis of measurements of magnetic fields shows that both model and real distributions can be approximated by a log-normal law with a mean log(B) =2.5. Based on this comparison we conclude that the magnetic flux variations of OBA stars on the main sequence (MS) are very slow. The shape of the magnetic field distribution for O, B and A stars appeared to be similar. This means that the mechanisms of the generation of their magnetic field are probably identical.