Abstract: The phase transformation and precipitation behavior of acicular ferrite in high-rise construction steel was observed in situ by high temperature laser confocal microscope.According to its nucleation site the ferrite can be classified into crystal interface,grain boundary edge and inclusions.The nucleation position and growth rate of acicular ferrite were analyzed theoretically. The following conclusions are drawn:1) the growth rate of grain boundary ferrite is inhomogeneous and the growth process deflects; 2) Three kinds of nucleation positions of ferrite and the sequence of ferrite precipitation were found; 3) finally,the growth characteristics of ferrite precipitated at different positions were revealed by in-site observation.