In the booming development of computer technology, countless counterfeit products are being forged in the market. Nowadays, many countries have adopted durable and high-security plastic substrates as identification cards or for product protection. The purpose of this study is to adopt digital halftoning technique to embed watermarks onto plastic substrates with self-authentication capability. By using halftoning algorithm with threshold matrices to form binary screen line microstructures with varied spaces, the watermark mask can be converted into an encrypted image via logical operations. Then the encrypted image is output on a plastic substrate. Finally, by bending and overlapping this plastic substrate, the watermark decryption and self-authentication effects can be achieved. The proposed method can be applied to many potential value-added applications and product protection.