Polycrystalline gallium nitride (GaN) thin films were deposited at low temperatures on GaAs (001) substrates via plasma-enhanced atomic layer deposition (PEALD). The growth process, surface mechanism and interface characteristics were investigated. The results show that the PEALD temperature window is 215~270 ℃, and the average growth rate of GaN thin films is 0.082 nm/cycle. The GPC analysis was performed in terms of kinetic energy barriers and thermodynamics. It is found that the GaN thin films are polycrystalline with hexagonal wurtzite structure and have a tendency to form (103) crystal. An amorphous layer of about 1 nm is observed at the GaN/GaAs interface, which may be related to limited active sites on the substrate surface before growth and the steric hindrance effect of the precursor. Most interestingly, in the deposited GaN films, all N elements combine with Ga elements to form GaN by Ga-N bonds, but a small part of Ga forms Ga-Ga bonds and Ga-O bonds. This bonding method during deposition may be related to the defects and impurities in the GaN thin films.