Abstract: The whole body counter is used to monitor the contamination of the human body and has a high requirement for the minimum detection limit (MDA). In order to have a better sensitivity for the whole body counter, it is necessary to reduce the background radiation and improve the radiation detection efficiency. Based on MCNP5, the energy response of the NaI detector in the geometric position is simulated. The position of the detector and the angle of the collimator are studied. At the same time, the shielded structure with different thickness is simulated and analyzed. The study shows that when the thickness of the shielded steel plate is 14.9 cm, the background radiation can be reduced to a large extent. In the source structure to detect the cavity collimator angle above 26, when the distance between the detector and the body mold is 20 cm, the detector can get the ideal energy spectrum response and detection efficiency.