A protection method for preventing a hard copy of a document from being released or reproduced is provided. The protection method comprises: selecting a pattern with a closed contour line and utilizing a level set method to process the pattern to generate at least a watermark; and embedding the watermark into the document. In addition, the protection method can further comprise: detecting the hard copy of the document to determine whether at least a watermark exists in the hard copy of the document; and forbidding releasing or reproducing the document when the watermark is detected in the hard copy of the document.