The invention discloses a formation pressure calculation method. The method mainly comprises the following steps: additionally mounting a displacement sensor on a throttle valve of a throttle manifold, and additionally mounting a differential pressure sensor on two sides of the throttle manifold; calculating parameters required for formation pressure calculation; adjusting the opening degree of the throttle valve to be at least three different opening degrees; reading a read number of the displacement sensor and a read number of the differential pressure sensor under the condition of each opening degree; calculating gas mass flow and volume flow under the condition of each opening degree; calculating a bottom hole pressure under the condition of each opening degree; and according to the gas volume flow under the condition of at least three different opening degrees and the bottom hole pressure under the condition of the corresponding opening degrees, calculating a bottom hole formation pressure. According to the method, the formation pressure under the condition that a well cannot be closed due to the fact that gas drilling encounters high gas flow or blowout can be determined, thereby providing a basis for subsequent construction scheme making and well killing method selection; and the method is suitable for field use.