Abstract: Sodium iodide (NaI) detector has wide applications in Prompt Gamma-ray Neutron Activation Analysis (PGNAA) due to its excellent detection efficiency and relatively low cost. In order to obtain the response of the detector to gamma-rays during PGNAA measurement process, a Monte Carlo model was established for the NaI detector to simulate the transport processes of neutrons and gamma-rays. Measurements and analyses were conducted on standard point sources of ⁶⁰Co, ¹³⁷Cs, ¹⁵²Eu, ¹³³Ba, and ²⁴¹Am, as well as a waterborne source of ¹³⁷Cs. These measurements were compared with MCNP simulation calculations, and the results demonstrated agreement between the two methodologies. Then, measurements were conducted on aqueous solutions using a D-D neutron generator, and the 2.223 MeV characteristic gamma-ray produced by the H element radiative capture reaction was analyzed. The research results show that the deviations between MCNP simulation calculations of characteristic peaks for the H element and experimental measurements were consistently within 13%, demonstrating that the NaI measurement system model established by MCNP can simulate the transport processes of gamma and neutrons, laying a foundation for the subsequent optimization of the system and other related work.