Abstract: Binary watermarking image is hidden by the method of mean-quantization.The principle ensuring that the watermark can be extracted blindly and correctly is analyzed. How the quantifying step size affects the properties of watermark is discussed,and theoretic inferior limit of the peak signal-to-noise ratio （PSNR) is estimated for the watermarked image.Experiments verified these theoretical analysis results.Computational complexity of the algorithm is reduced and real-time manipulation is feasible.