This invention is used for oil field to calculate flow pressure of a machine-pumped oil well. Its main technical feature is that when waveform of dynamic liquid level signal is "sawtooth wave", or "reverse trapezoid wave", it respectively indicates that there is or not a froth section above the dynamic liquid level. By measuring the interval between the trailing edge and rising edge of trapezoid wave and substituting it into a regression equation, the length of froth section can be found out, thereby eliminating the influence of froth section to calculating precision, which existed when liquid level method is used to calculate the flow pressure of machine-pumped oil well. |