Provided is a method of depositing an amorphous silicon thin film by chemical vapor deposition (CVD) to prevent bubble defect occurring when an amorphous silicon thin film is deposited on a substrate contaminated by air exposure. The deposition method includes cleaning a surface of the contaminated substrate with a reaction gas activated by plasma and depositing an amorphous silicon thin film on the cleaned substrate. Here, a vacuum state is maintained from the substrate cleaning step to the thin film deposition step in order to prevent contamination of the surface of the cleaned substrate by re-exposure to air.