Abstract: A 4πβ-4πγ detector configuration with a 4πβ （PC） flow proportional counter and a large size welltype 4πγ NaI(Tl) crystal was described, and the theoretic function of sum efficiency extrapolation was established. The radioactivities of^60Co and 134Cs source were measured using this sum extrapolation method, 0.29% and 0.19% relative uncertainty obtained for each, and the result was compared with the value of coincidence efficiency extrapolation. The analysis indicates this method is fit for measuring the activity of complex decay nuclides.