The invention provides high-strength low-carbon low-alloy steel for building and a preparing process thereof, and relates to the technical field of alloy steel. The alloy steel is prepared from, by weight percentage, 0.05-0.11% of C, 0.1-0.17% of Si, 0.7-1.1% of Mn, 0.6-0.85% of Ni, 0.01-0.025% of B, 0.1-0.18% of Zr, 0.06-0.09% of Nb, 0.001-0.003% of Re, 0-0.006% (including 0.006%)of P, 0-0.01% (including 0.01%) of S, and the balance Fe and inevitable impurities. The preparing process mainly comprises the steps of smelting, pouring, forging, quenching, tempering and the like. All elements in the alloy steel are reasonable in proportioning. Moreover, through the optimized preparing process, the prepared low-carbon low-alloy steel is high in strength and tenacity, has well-closed formation,is free of defect, and has high corrosion resistance and abrasive resistance. As a construction material for building engineering with excellent performance, the high-strength low-carbon low-alloy steel has a great using effect.