Abstract: The main noise of γdigital radiography(γDR) system is the statistical noise caused by the randomness of interaction between γ ray and matter. It's an efficient method to reduce the noise by using average value of sampling signals. The factors which influence the noisereducing effect of addup method is deduced theoretically, and relationship between noise intensity and addup times and sampling period is acquired, and the result is verified by experiment. The result has been successfully applied in the noise reduction of Co60 container inspection system. 