The invention discloses a low-alloy superhigh-intensity diphase steel and the heat treatment method, which is characterized in that: the components of the diphase steel (wt %) are C of 0.3 to 0.7, Si of 0.01 to 3.0, Al of 0.0 to 2.0, Nb of 0.0 to 0.25, V of 0.0 to 0.3, Mo of 0.0 to 2.0, Ni of 0.0 to 4.0, Mn of 0.05 to 3.0, Cr of 0.00 to 3.0, Co of 0.00 to 2.0, W of 0.0 to 2.0, S less than 0.04, P less than 0.04 and Fe of balance; the heat treatment method is that: a workpiece is first heated to 800 to 1000 degree centigrade to process the austenite treatment, and then the workpiece is quickly quenched into a liquid quenching medium of 50 to 250 degree centigrade, and then the workpiece is partitioned in a liquid quenching medium of 250 to 450 degree centigrade, and then the workpiece is quickly quenched into a liquid quenching medium of 100 to 250 degree centigrade for holding, finally the workpiece is quenched into water, and then the workpiece has a three-phase organization with martensite, nanometer ferrito martensite and remaining austenite with rich carbon. The low-alloy superhigh-intensity diphase steel and the heat treatment method has an advantage of increasing the intensity and plasticity of workpieces.