The solidification microstructures of Pb-45%Sn hypoeutectic alloy in rotating magnetic field (RMF) were investigated. When the rotating frequency keeps constant，the grain size reduces linearly with the increase of magnetic intensity，and structure of the primary phase changes from the dendrite to spherical crystals. X-ray diffraction (XRD) measurement indicates that the RMF causes the lattice expansion of primary phase (Pb)，and the lattice parameter increases at first and then decreases with the increase of RMF intensity. Therefore，there is a critical value of magnetic intensity. Moreover，the Sn content in primary phase (Pb)reduces gradually with the increase of magnetic intensity. The results are analyzed theoretically based on electromagnetism field theory and scattering law. It is revealed that RMF causes severe fluid flow，and accelerates the diffusion of solute atom and heats the melt，which results in the increase of nucleation rate and the decrease of growth velocity of crystal.