Abstract: The moisture response of the powder coal using an insert-type neutron gauge is calculated with the neutron diffusion theory.The neutron gauge is composed of a point neutron source(~241Am-Be) and a disk type detector(Li glass scintillator).The packing variety of density ρ_i for the powder coal with the volume assumes V form.At fixed packing,the moisture response can be revised by ρ_i~-x,in this text x=1.85.The calculation results indicate that the thermal neutron flux density increases with the moisture in short source distance and it is contrary in long source distance.When the source distance is 10 cm,the moisture response is linear,and its linear correlation coefficient is 0.999 5.The accuracy of theoretical moisture is 0.08%.