A 12CrNi2 alloy steel, which was composed of a major ferrite and minor austenite, has been fabricated by means of the laser additive manufacturing technology. The microstructure of distinct interlayer heat-affected zone consists of coarse ferrite and a small amount of austenite comparing to the microstructure of inner deposited layer. The microstructure of as-deposited steel was homogenized after isothermal quenching treatment, and meanwhile the strength was improved significantly. The evolution process of the microstructure and morphology of the bainite with the isothermal quenching temperature and time and its effect on the yield strength and tensile strength as well as the elongation of the alloy steel are reported. And the optimized heat treatment process with good comprehensive mechanical properties is proposed.