The pipe holdup measurement is very important for decommissioning nuclear facilities and nuclear-material control and accounting. The absolute detection efficiencies (ε_sp) 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 Al 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 (U_0.95rel=5.4%) and 216.2mg (U_0.95rel=3.2%), respectively, which are in accordance with the reference values.