Disclosed are an organic anti-reflective coating polymer having a structure represented by the following formula I which is introduced to the top portion of photoresist, its preparation method and an anti-reflective coating composition, in a process for forming ultra-fine patterns of photoresist for photolithography by using 193 nm ArF or 157 nm VUV light source. More particularly, the present invention provides an organic anti-reflective coating polymer capable of protecting a photoresist from amine to improve the stability of a post exposure delay and to minimize pattern distortion caused by a swing phenomenon during a patterning process, its preparation method and an anti-reflective coating composition comprising the same. [formula I] wherein each of m and n is an integer ranging from 5 to 5,000.