The applicability and parameters of the image method for solving the thermal neutron leakage distribution on the plane surface of the semi-infinite hydrogen-containing medium in the case of the neutron field originating from a point source of fast neutrons is investigated. The research performed has shown that for the use of the image method it is necessary to introduce not only the concept of the effective extrapolated boundary but also the concept of an effective origin of the point source of fast neutrons. The values of effective extrapolated length and effective location of the point source were calculated and examined by experiment for pure water.