The work aims tostudy and analyze the process optimization of deposition of ultra-thin Al2O3 barrier layer on PET surface by plasma-assisted atomic layer deposition (ALD). The deposition rate (GPC) of Al2O3/PET film was optimized by single factor combined with response surface design method. The growth mode and surface roughness of the film were obtained by AFM analysis. Finally, the barrier property of the film was characterized by water vapor transmission rate. According to the experimental results, the deposition parameters of the largest GPC were:TMA pulse time 0.17 s, purge time 11 s, O2 pulse time 0.35 s, purge time 10 s, deposition rate 0.215 nm/cycle; the film grew in a layered growth mode with a rough surface. The root mean square (RMS) was 0.58 nm; the water permeability decreased from 7.456 g∙d/m2 of the original film to 0.319 g∙d/m2 after 500 cycles of deposition. The preparation parameters of ALD are optimized by response surface methodology, and the water vapor barrier was increased by 25 times when Al2O3 is deposited on PET surface at about 100 nm.