The present invention discloses a method and system for calculating SUV normalization coefficient in a SPECT quantitative tomographic image, comprises the following steps: a) averaging the anterior plain film image and posterior plain film image obtained by performing plain film scanning on a subject's whole body to get an average image, from which the area corresponding to the radioactive uptake site in the subject is segmented, b) correcting attenuation coefficient of human body, detection efficiency and acquisition time for each pixel value in the effective radioactive uptake area within said average image, c) averaging all the effective radioactive uptake area pointed at by said quantitative two-dimensional radioactive uptake activity image obtained in step b, and performing volume normalization and decay time correction to figure out the average radiopharmaceutical activity concentration absorbed in the subject.