In this paper, we describe an improved semi-fragile watermark scheme, which use the density of edge points in each block of an image to determine the watermark strength in watermark embedding process, so it reduces the false positive and miss positive of original algorithm effectively. We use Hilbert scan curve to simplify the computations of correlation in watermark detection process. The tamper detector can identify the tampered region from the received watermarked image without original watermarked image, which can be used for the content authentication. Experimental results show that this scheme is robust to incidental manipulations for image, such as JPEG compression.