The theory and method of b0 values of phengites regarded as a relative geobarometer have been valued by geologists of the world. There is no research on polymetamorphism reflected by b0 values of phengites reported in the world today. We think that the populations of b0 values must be distinguished in the study of b0 values of phengites of low grade metamorphic rocks occurred in ancient orogenic belts, especially, for the polycyclic orogenesis in China. In this paper, the authors put forward a method of distinguishing polymetamorphism by statistical analyses of b0 values of phengites on the basis of research on phengites of Lancang Group. Statistical analyses of 486 b0 values of phengites show that there are medium-and high-pressure metamorphisms in Lancang Group. The intersection of two straight lines representing bi-nomal distributions is situated at the point corresponding to b0=9.040Å in probability graph. The mean b0 value of high-pressure is 9.047, the standard deviation and cumulative probability are 0.005 and 55% respectively. The average b0 value of medium-pressure metamorphism is 9.027Å, its standard deviation is 0.010. The cumulative frequency curve of medium-pressure metamorphism is situated between cumulative frequency curves of Ryoke and Easterm Alps. The cumulative frequency curve of high-pressure is similar to that of Sanbagawa.