Abstract: As a high-energy neutron source, the D-T neutron generator has been used in the active detection technology of special nuclear materials. The time coincidence method can be used to achieve multi-mode imaging of the measured object, and the beam spot size of the neutron source is a very important factor that affects the imaging position resolution. Therefore, combined with the characteristics of D-T reaction and practical application environment, a method for n-α correlation coincidence measurement to determine the beam spot size of neutron source was developed. This method was used to measure the beam spot size of a small mobile neutron generator. The beam spot size obtained is （2.8±0.9） mm, which is consistent with the measurement result of about 3 mm obtained by direct observation with CCD camera, which proves that this method is feasible. This method can also be used to assist D-T neutron source beam tuning and beam spot size monitoring in associated particle imaging experiments.