Abstract: This paper studies the optimum detection model of the multiplicative spread-spectrum watermark embedded by fixed strength.The parameters of the distribution models for optimum detections in DFT,DCT and DWT domains are estimated in global range and local range respectively,and the optimum detection performances of the two parameter estimation methods are compared.Experimental results show that the performance of local parameter estimation is superior to that of global parameter estimation when there are no attacks,and the performance of local parameter estimation is worse than that of global parameter estimation when there are attacks.Additionally,the JPEG attack leads to the increase of the false positive probability,and different attacks have different effects on the distribution model.