According to an orthogonal experimental design and neural network prediction models, a new kind of backfill cementitious materials of unclassified tailings-rod milling sands is prepared with unclassified tailings partly replaced rod milling sands as aggregates, iron slag powder as the active materials as well as desulphurized ash and quicklime as the main stimulator. Hydration products and microstructures of the backfill cementitious materials are analyzed by XRD and SEM. The results show that with addition of 30% unclassified tailings, the compressive strength of the backfill cementitious materials cured for 3 days, 7 days and 28 days reaches 1.73 MPa, 4.22 MPa and 6.93 MPa, and the corresponding increment is 8.13%, 51.8% and 34.0% respectively in comparison with the ordinary cement, and these strength values can meet the backfill strength index of Jinchuan Nickel mine. The hydration products of the new backfill cementitious materials are mainly C-S-H gel and flocculent CSH gel, which form dense gel and bond the aggregates together resulting in the high mechanical strength of the materials. Therefore, utilization of 30% unclassified tailings can be achieved for the nickel mine company through producing this new backfill cementitious materials.