The invention discloses low-alloy, high-strength and high-toughness composite phase steel. The low-alloy, high-strength and high-toughness composite phase steel comprises the following components in mass percentage: 0.40-0.90% of C, 0.5-2.0% of Si, 0.5-3.0% of Mn, 0.5-4.0% of Cr, 2.0-5.0% of Al, less than or equal to 0.01% of S, less than or equal to 0.01% of P and the balance of Fe. The invention further discloses a preparation method of the low-alloy, high-strength and high-toughness composite phase steel, comprising the following steps of: firstly, quickly heating steel to 850-1000 DEG C, and keeping the constant temperature for 2-30min so as to austenitize a workpiece sufficiently; secondly, quickly quenching to -20 to 50 DEG C so as to obtain a partially supersaturated martensite and an austenite without martensitic transformation; thirdly, keeping the constant temperature of the workpiece at 300-600 DEG C for 5-600min for carbon distribution; and finally, quenching to room temperature to obtain the composite phase steel. The preparation method is simple in process; by the preparation method, steel strength is improved, good toughness is ensured, and the low-alloying cost is low; and the preparation method is wide in industrial application prospect.