Multiferroic materials, which show simultaneous ferroelectric and magnetic ordering, exhibit unusual physical properties. Bismuth ferrite is a room temperature magnetoelectric multiferroic material. Therefore its applications and physical properties are very important. In this thesis, we cooperate with Professor Y. H. Chu’s group. We requested them to provide good bismuth ferrite films. D. S. Rana’s group in the experiment, which is THz emission by ultrafast laser, observe phenomenon of ultrafast depolarization, and they consider the phenomenon is about electron-lattice interaction. Using ultrafast pump-probe spectroscopy, we measure the transient reflectivity change ΔR/R of the material as a function of a pump-probe delay at various temperatures. We can observe the ultrafast dynamic behaviors of electron, lattice and spin, and we can understand complex physical mechanism in this material. And we can observe the phenomenon of ultrafast depolarization.