In this paper, fist'we discribe briely neutron diffusion equation in three-dimensional segion, establishment of its finite difference equation and inner and outer iteration procedures. Then we derive in detail the SIP (Strongly Implicit Procedure) and SSIP (Symmetrical SIP) for solving finite difference equation in inner iteration procedure, accelerated method for inner and outer iteration and selection of iteration parameters. Finally, some results of nummerical experiment are presented.