A power conversion system and a magnetic component thereof are provided. The magnetic component includes a magnetic core assembly, two windings and an air gap. The magnetic core assembly includes two substrates, four winding pillars and an auxiliary pillar unit. The four winding pillars and the auxiliary pillar unit are located between the two substrates. A connecting line of centers of the winding pillars forms a quadrangle which has a first diagonal line and a second diagonal line. One winding is wound around two winding pillars on the first diagonal line, and magnetic fluxes on these two winding pillars have the same direction and amount. The other winding is wound around the other two winding pillars located on the second diagonal line, and magnetic fluxes on these two winding pillars have the same direction and amount. The directions of the magnetic fluxes on the two neighboring winding pillars are opposite.