In the past few years, perovskite film has been considered as a promising materials for solar cell devices due to its outstanding performance. To maximize the perovskite solar cell performance, it is necessary to understand the crystallization mechanism of perovskite film. In this study, we monitored the crystallization and decrystallization of the lead halide perovskite (MAPbI3-xClx) film under thermal annealing and UV-laser exposure processes by using in-situ terahertz time-domain spectroscopy. The strength of vibrational resonances in THz frequency range is found to be a good indicator of perovskite crystallinity. We measured the THz spectra as we annealed the perovskite film at various temperatures in order to achieve the degree of crystallization, i.e., the transition of perovskite structure from the intermediate phase to the tetragonal phase. In addition, we investigated the UV-laser-induced phase transition of the perovskite film. Because it is widely known that UV light illumination on perovskite film tends degrade the perovskite cell efficiency, its influence on the crystallization is our primary concern. Surprisingly, the crystallization phase increases for 10 min, until it starts to degrade over a couple of hours. We also studied the transient transport properties of the films under UV illumination. The correlation between the degree of crystallization (obtained from THz transmission) and the transport parameters exhibited the electric percolation threshold behaviors in the perovskite films. These information are expected to be crucial for optimizing the fabrication method of perovskite solar cell.