The multibeam concept for the accelerator-driven subcritical reactor (ADS) has advantages in the power distribution of the core, and it could reduce the requirement of proton beam current intensity for each beam. In comparison with the single-beam concept, the multibeam concept could also reduce the thermal load of the beam window significantly. This paper focuses on the study of external source effects for different multibeam concepts for an ADS for nuclear waste transmutation (ADS-NWT). The different multibeam concepts include the three-beam, four-beam, six-beam, and seven-beam concepts for the ADS-NWT. By using the calculation tools FLUKA and SuperMC with the nuclear data library ENDF/B-VII.1, the variations of the keff and total power, as the function of the position of the spallation targets, are provided for each multibeam concept. The results show that the keff and total power were affected by an interference effect between the spallation targets. For the transport of fission neutrons in the core, the maximum radius of the interference effect between the spallation targets was 40 cm. Considering the transport of spallation neutrons in the ADS-NWT, the maximum radius of the interference effect between the spallation targets was 60 cm. The spallation targets were moved from the inner circle to the outer circle of the fuel zone, and the different variations in keff and total power trend for the three-beam, four-beam, six-beam, and seven-beam concepts for the ADS-NWT were obtained.