Abstract: The moisture response of the lump coal for the insert-type neutron gauge is calculated using the neutron diffusion theory. The neutron gauge is composed of a point type of neutron source (241Am-Be) and a surface in short source distance detector (Li glass). The calculation results indicate that the thermal neutron flux (φ3(r))density increases with the moisture increases in short source distance detector, and it is contrary to the long source distance. When the source distance is 10 cm, the moisture response is linear(R>0.993 0).