High-entropy alloys (HEAs), defined as solid solution alloys which have at least 5 principal elements but no more than 13 elements, with concentrations of each principal element ranging from 5% to 35% in atomic fraction, are emerging as one of the hot research frontiers in the metallic materials field. The significance of HEAs originates from their various combinations of high strength, good thermal stability and excellent resistance to corrosion, wear and oxidation. HEAs exhibit simple solid solutions with bcc and/or fcc structure(s) due to the effect of high mixing entropy in the solid solution state of HEAs, which may make the HEAs with improved mechanical and physical properties. However, a small quantity of intermetallic compounds can also form in certain HEAs, indicating that the formation of simple solid solutions cannot solely depend on the high mixing entropy. Then, the theory of HEAs based on the concept of entropy-enthalpy competition to judge whether or not simple phases will form was proposed. However, even if an alloy meets these criterions, it can still contain intermetallic phases. Why and how these intermetallics form in HEAs needs much more clarification. In this work, Co-Al-Cu-Ni-Mo_x (x=0, 0.5, 1) powder system with close-to-equiatomic ratios was mixed and laser surface alloyed onto 2Cr13 stainless steel substrates, and then the FeCoCrAlCuNiMo_x HEA coatings were obtained by reaction synthesis of Fe, Cr with Co-Al-Cu-Ni-Mo_x powder. The phase transition mechanism, microstructure and microhardness of FeCoCrAlCuNiMo_x coatings were investigated by XRD, SEM, EDS and microhardness tester. Experimental results showed that the principal elements of Fe, Cr in 2Cr13 stainless steel substrate participated in surface alloying process during the laser irradiation, forming FeCoCrAlCuNiMo_x laser high-entropy alloying coatings. With the increase of Mo content, the crystal structures of FeCoCrAlCuNiMo_x laser high-entropy alloying coatings evolved from fcc+bcc two-phase solid solution to fcc+bcc solid solution with hcp phase precipitations. The hcp phases were mainly Ni₃Mo and Co₇Mo₆, and the content of Ni₃Mo phase was higher than that of Co₇Mo₆. The phase formation analysis indicated that besides Ω and δ parameters, solidification temperature of the molten pool must be considered during the phase selection, instead of melting point as suggested previously. The microstructure of the coatings exhibited a typical dendrite structure. With the increase of Mo content, the block-shaped Ni₃Mo and Co₇Mo₆ precipitated in the innerdendritic regions. The microhardness of the FeCoCrAlCuNiMo_x laser high-entropy alloying coatings was 390~490 HV, which significantly increased with the increase of Mo content.