Abstract: La_0.5Ca_0.5MnO₃(LCMO)thin films were prepared by the pulsed laser deposition method and then annealed in situ under different high magnetic fields. The microstructures and magnetic properties of the films were characterized and measured by XRD,FESEM and SQUID. The results show that an elongation in out-of-plane lattice parameter,a dramatic increase in grain size,and a decrease in low-temperature magnetization can be observed in the LCMO films annealed in high magnetic fields. The evolution mechanism on grain size is investigated by establishing a rate equation of grain growth,including the effect of a high magnetic field. It is argued that as an extra driving force of grain growth,the introduction of a high magnetic field leads to a lowering in critical grain size in LCMO films,favoring the grain growth. In addition,an antiferromagnetic-ferromagnetic core-shell model based on phase separation is proposed to understand the evolution of magnetic configuration. It is proposed that the increase in grain size results in a decrease in volume fraction of ferromagnetic phase,and thus a decrease in low-temperature magnetization.