In a single impurity Anderson model,with equation of motion and Green function,we study spin polarized transport through a quantum dot side-coupled to ferromagnetic leads and a mesoscopic ring.It shows that the coupling strength between quantum dot and mesoscopic ring,spin polarization,relative angle of magnetic moment in ferromagnetic leads and temperature are all important parameters for electron transport in the quantum dot system.It can be used to generate a spin-valve effect.It provides theories for a quantum dot system as a new material of spinning electrons.