Calculation methods of under balanced formation pressure under existing technology and its defects are introduced. Down hole pressure collection joints are development. Air/liquid double phase pressure down the well was measured during well shut off through in situ test. A new formation pressure calculation method was established based on conventional well test theory and formation transfusion theory and using measured data. The method was established on pressure balance around well wall and the down hole pressure at balance point which was actually measured. It eliminated the errors caused by unstable casing and stand pipe pressure and imprecise drilling fluid density. It can precisely calculate the formation pressures along the borehole. The calculated pressure by the method is much closed to actual pressures. The calculation errors of well shut off pressure were down about 4%.