1. A method of making a watermark with hybrid halftone dots, comprising the steps of: combining an amplitude modulation (AM) and a frequency modulation (FM) halftone dots through a hidden-pattern mask; using the hybrid halftone dots to make a printing with the watermark; deciding a density of the AM halftone dot first, then using a calibration chart to choose a corresponding density of the FM halftone dot; pre-processing the hidden-pattern mask to achieve a boundary of a combined image to be smooth; making a secured document by outputting the watermark in a shadow image whose density of AM/FM halftone dots is forming a hidden pattern becomes visible after duplicating; generating hidden patterns of different serial number to achieve a document-version control; and detecting a hidden watermark by simple tools.