The work aims to prepare Fe-based composite coating in situ on Q235 steel with argon arc as a heat source. With iron powder, Fe-based alloy powder (G302) and B4C powder as preset alloy powders, Fe-based composite coating was synthesized in situ on Q235 steel by tungsten inert gas cladding at two powder ratios of Fe:B4C=15:1 (mass ratio) (denoted as S1) and G302:B4C=8:1 (mass ratio) (denoted as S2) . Structure property was characterized in a series of methods. Microstructure of coatings was analyzed with scanning electron microscope, phase composition of coatings with X-ray diffractometer and microhardness variation with MHV-2000 digital microhardness tester. Interfaces between the coatings and substrates in sample S1 and sample S2 exhibited good metallurgical bonding property, and free from pores, cracks and other defects. XRD analysis showed that preset alloy powders reacted adequately and no residual B4C was left in coatings. Reinforced phase in sample S1 was mainly composed of Fe2B, Fe7C3 and Fe3C while main phases in sample S2 were composed of Cr-Fe, (Cr,Fe)7C3 and (Fe,Cr)2B. Borides and carbides in sample S1 were in noncontinuous network-like distribution, network-like structure in sample S2 weakened, and block or short rod-like structure appeared. Microhardness of substrate to coating in sample S1 and sample S2 was subject to gradient change, microhardness of coating was obviously higher than that of the substrate. Microhardness of the coating in sample S2 was up to 1200HV0.1, about 6 times as high as that of substrate. Fe-based composite coatings can be synthesized in situ using the above prefabricated alloy powders. The increase of boron and Cr element content can improve boride morphology to transform boride from network-like structure to block structure. Reinforced phase of in-situ synthesized composite ceramic is distributed uniformly in the iron substrate, hence hardness of the prepared coatings increases significantly.