Abstract: The phoswich β-γ coincident detector is key equipment for weak radioxenon isotope detection. Xenon will be absorbed on the surface of plastic scintillation (BC404) during measurement, and the absorption will make the detection background increase resulting in severe influences to measurement for next samples of weak-radioactivity. Neutron activation analysis (NAA) method is popular in trace analysis field because of its high sensitivity and nondestructive characteristic. In this paper, quantity of absorbed xenon was determined based on MCNP simulation on the γ-ray peaks of ¹³³Xe and ¹³⁵Xe produced by (n, 2n) reaction. The results show that NAA is a feasible method for measurement xenon absorbed on BC404 surface. The relative standard uncertainty of value is about 22%. Some suggestions for suppressing the standard uncertainty of the data are also proposed according to the source of uncertainty.