Abstract: The background neutron from target materials and the scattered neutron background within the accelerator hall of the 3 MV tandem accelerator neutron source at Sichuan University were systematically investigated. The energy and cross-section variations of neutrons emitted from (p, n) and (d, n) reactions on commonly used neutron target deuterium (tritium) absorbing materials (Ti, Zr) and substrate materials (Mo, Ta, Cu) were theoretically analyzed using nuclear reaction kinematics and the TALYS program as a function of proton (deuteron) energy. Experimental measurements were also carried out to determine the variation in neutron intensity as protons (deuterons) of varying energies bombarded different targets (tritium or deuterium targets, empty targets, and metal targets such as Ti, Zr, Mo, etc.). The results indicated that the background neutron intensity from protons (or deuterons) interacting with absorbing materials (Ti, Zr) and substrate materials (Mo, Ta, Cu) increases with energy, which severely limits the monochromatic neutron energy region of this accelerator source. The shadow cone method was employed to measure the scattered neutron background distribution at 1.4, 2.4 and 3.6 m from the neutron source, and the results were compared with Monte Carlo simulations. It was found that the scattered neutron background in the accelerator hall was approximately uniformly distributed, with interference from scattered neutrons diminishing as the distance to the neutron source decreased. These results are of significant importance for conducting precise neutron reaction cross-section measurements and high-level neutron physics research using this accelerator.