The present invention provides a new supported catalyst for olefin polymerization prepared by reacting a novel transition metal compound on a cocatalyst-supported support, in which the transition metal compound is coordinated with a monocyclopentadienyl ligand to which an amido-quinoline group is introduced, a method for preparing the same, and a method for preparing a polyolefin using the same. The transition metal catalyst compound used in the present invention is configured such that an amido group is linked in a cyclic form via a phenylene bridge. Thus, a pentagon ring structure of the transition metal compound is stably maintained, so that monomers easily approach the transition metal compound and the reactivity is also high. Accordingly, if the supported catalyst for olefin polymerization prepared according to the present invention is applied to the copolymerization of ethylene and monomers having large steric hindrance in a slurry or a gas phase process, a linear, a linear, high molecular weight polyolefin having a very low density of 0.900 to 0.970 g/cc or less can be obtained.