The present invention relates to a catalyst for hydrocarbon steam cracking, a method of preparing the same, and a method of preparing olefin by the hydrocarbon steam cracking by using the catalyst, and more specifically, to a catalyst for hydrocarbon steam cracking for preparing light olefin including an oxide catalyst (0.5≦j≦120, 1≦k≦50, A is transition metal, and x is a number corresponding to the atomic values of Cr, Zr, and A and values of j and k) represented by CrZrjAkOx, wherein the composite catalyst is a type that has an outer radius r2 of 0.5R to 0.96R (where R is a radius of a cracking reaction tube), a thickness (t; r2−r1) of 2 to 6 mm, and a length h of 0.5r2 to 10r2, a method of preparing the same, and a method of preparing light olefins by using the same.