The pipe holdup measurement is very important for decommissioning nuclear facilities and nuclear-material control and accounting. The absolute detection efficiencies (εsp) of full-energy γ rays peak under different source density distribution function have been simulated using the Monte Carlo (MC) software, and the counting rates (n0) of the characteristic γ rays have been measured using the γ spectrometer followed by the calculation of the holdup. The holdup is affected by the energy of γ rays, distance at which they are detected, pipe material, thickness,and source distribution of pipe, especially source distribution at a short distance. The comparative test of 235U reference materials on the inner wall of Fe and A1 pipes (the total mass of 235U is 44.6 mg and 222.8 mg, respectively) have been accomplished using this method. The determined result of 235U is 43.2mg (U0.95rel=5.4%) and 216.2mg (U0.95rel= 3.2%), respectively, which are in accordance with the reference values.