The reliable watermarking algorithm has been optimized by DD-PCA (Double-Decomposition Principal Component Analysis) using the relations and characteristics of PCA and SVD (Singular Value Decomposition). The principal component of the watermark is extracted using SVD and embedded into the singular values of the cover image. Then the second decomposition is used creatively. Experimental results show the algorithm proposed is greatly imperceptible-the PSNR can be over 40dp though embedding factor is 40. The algorithm is robust to noising, compression, blurring and geometric attacks. Besides, experimental results also show high reliability in solving the false positive.