21 amples of phengite used for present work were collected from 4 low-temperature and high-pressure metamorphic belts in China. The chemical compositions, b0 value and Fe2+ site occupancy have been measured by means of electron microprobe,X-ray powder diffraction and Mossbauer spectrometer. The ratio, R=Fe2+ (M1)/Fe2+(M2), is used to describe Fe2+ order degree in phengite. b0 vs R diagram has been established, in which there are 3 regions corresponding to low-, medium- and highpressure. This is a new geobarometric indicator. Mechanism of Fe2+ order at M2 site of phengite have been discussed. Progress of Fe2+ order at M2 site of phengite is suggested:high metamorphic pressure→Tschermarks substitution → (1) Si4+ replace tetrahedral Al3+, more Si enter phengite structure, (2) (Mg, Fe) replace Al3+ at M2site, which results in Fe2+ order at M2.