The effects of 0.01–0.11 wt.% Zr on the inclusions, microstructure, tensile properties, and impact toughness of the China low activation martensitic steel were investigated. Results showed that Zr exhibits good deoxidation and desulfurization abilities. The scanning electron microscope was used to examine the inclusions in the ingots. The main inclusions in the alloys were Zr–Ta–O, Zr–O, and Zr–O–S. However, some blocky Zr-rich inclusions appeared in Zr-2 and Zr-3 alloys. Typical martensitic structures were observed in the alloys, and average prior austenite grain sizes of 21.1, 15.7, and 14.8 µm were obtained for Zr-1, Zr-2, and Zr-3 steels, respectively. However, increasing Zr content of the steels deteriorated their mechanical property, owing to the blocky inclusions. The alloy with 0.01% Zr resulted in excellent mechanical properties due to the fine inclusions and the precipitation of Zr₃V₃C carbides. Values of 576 and 682 MPa were obtained for the yield strength and ultimate tensile strength of Zr-1 alloy, respectively. Furthermore, the ductile–brittle transition temperature of the alloy decreased to − 85 °C.