Based on the object-oriented C++ language, a 3D cylindrical coordinate conformal grid generation program is developed. The conformal grid generation of beam-field interaction(BFI) device discretizing is performed, providing integral line and face elements for Particle-in-Cell simulation algorithm. By defining the basic elements of the three-dimensional cylindrical coordinate grid system, including grid step-size, grid index, guardian grid layer and bounding box, the spatial information of the model can be converted into the information of the cylindrical coordinate grid information necessary for numerical calculation. The grid cells on-axis should be specially treated so as to maintain the consistency of the particle-in-cell algorithm between the axial grid and the non-axial grid. The discrete boundary points on the model subsurface and on the model edge were attained by ray-tracing algorithm. Meanwhile, the vertices of the model were obtained by topological relations. Topological information and cylindrical coordinate grid information of the two types of boundary points and model vertices were recorded, then the basic grid elements were coupled with the boundary point information, finally the model was reconstructed in the discrete grid system. Taking the relativistic magnetron for example, using the conformal grid generation technique proposed in the paper to discretize the model, the transparent cathode, anode, and resonant cavity structures of the magnetron can be identified.