Abstract: Pressure factor is a basic parameter for petroleum reservoir evaluation, which can be used to analyze the pressure status of reservoirs. However, in an area or reservoir with uneven surface and/or high hydrocarbon column, the calculation of pressure factor may lead to result with large deviations. This paper studies the reasons for the phenomena above. For the area with uneven surface, it is the calculation of hydrostatic pressure that leads to the deviations, which can be eliminated by replacing the depth from surface with depth from water level. For the reservoir with high hydrocarbon column, it is the mischoice of calculation datum depth of pressure factor that results in deviated pressure factors, which can be eliminated by using the middle depth rather than the top or bottom depth of hydrocarbon column. The intercept pressure of hydrostatic pressure-depth curve, a surplus pressure of reservoir fluids flowing from underground to surface, is defined as spontaneous production factor of reservoir fluids. The larger the spontaneous production factor, the stronger the ability of reservoir fluids to flow spontaneously