The invention discloses molybdenum-strengthened 10Ni7Co secondary-hardened ultrahigh-strength steel and a preparation method thereof and belongs to the technical field of alloy steel. The molybdenum-strengthened 10Ni7Co secondary-hardened ultrahigh-strength steel comprises the following chemical components in percentage by weight: 0.20-0.45 percent of C, 0.5-3.5 percent of Cr, 7.00-12.0 percent of Ni, 5.00-8.00 percent of Co, 1.0-4.0 percent of Mo, less than or equal to 0.30 percent of V, less than or equal to 0.20 percent of Nb and the balance of Fe and inevitable impurities. The alloy can be prepared by adopting processes of vacuum induction and vacuum consumable remelting or the processes of vacuum induction and electroslag remelting smelting. Compared with the prior art, the molybdenum-strengthened 10Ni7Co secondary-hardened ultrahigh-strength steel has the advantages that the ultrahigh-strength steel has good comprehensive performance, ultrahigh strength, high ductility and toughness, excellent heat treatment process performance, high temper resistance and over-aging resistance, low quenching temperature and good grain size and texture; the content of cobalt element which belongs to strategic resources is saved, and the economical efficiency is improved.