Literature[1] has created a very useful area coordinate,but not well understood the link between the area coordinate system and some of the original coordinate system.In addition,there appear some errors like＂if the u1＋u2＋u3=0,that is（u1：u2：u3）represent a infinity points＂.This article wants to analyze these problems and gets conclusion,area coordinate system is a special kind of projective coordinate system.