A method of watermark with hybrid halftone dots that is suitable for a printed shadow image. The watermark patterns that are composed of amplitude modulation (AM) and frequency modulation (FM) halftone dots (their concentration has been calibrated) cannot be perceived beyond a certain distance by the human eye. This is done through outputting the density calibration chart to choose the proper density parameters, which are applied to the manufacture process of the watermark with hybrid halftone dots. The hidden patterns of a watermark need to be pre-processed first, and thereby making the image blocks of AM and FM halftone dots to be combined smoothly. The copy machine, the lenticular lens and the optical scanner can detect the hidden watermark in the printed shadow image. Each secured document possesses a different serial number to achieve the document-version control.