Abstract: Formaster-FП phase transformation instrument was used to determine the change curve of the dilation of 5%Ni low temperature pressure vessel steel with temperature at different cooling rates. The critical transformation points were measured to be A_c1=640 ℃ and A_c3=753 ℃. The continuous cooling transformation（CCT） curve of undercooling austenite of the tested steel was plotted by using Origin software. The results show that during the continuous cooling transformation of austenite, the phase transformation of diffused polygonal ferrite is promoted by a small cooling rate of 0.2 ℃/s. With the increase of cooling rate, the phase transformation of polygonal ferrite and the critical phase transformation temperature decrease, while the transformation amount of bainitie increases. With the increase of cooling rate（reaching 5 ℃/s）, the high-temperature austenite rapidly passes through high temperature phase change region, which transforms into martensite at low temperature region.