PVDF has a great potential to be a core material for designing flexible pressure sensors fit on the human body. However, the piezoelectric constant of PVDF is low and the sensor sensitivity is limited. In order to study the piezoelectric properties of PVDF films when applied to pressure sensors, a piezoelectric simulation model is established using COMSOL to calculate the potential, surface charge density and stress generated by PVDF film under a low-pressure load in steady state. The correctness of the simulation method and model is verified by comparing the simulation results with the piezoelectric equation calculation results. It is found the stress generated in the length and width directions of PVDF film using rigid substrate weakens the piezoelectric properties, then a structural optimization method is verified to improve piezoelectric properties by reducing the length and width of PVDF films. In addition, the stress state of PVDF film using rigid and flexible substrate is compared, and it is found that the stress state of flexible substrate can lead to significantly higher piezoelectric output. This research combining simulation method with piezoelectric theory, can provide reference for other basic research of PVDF materials in terms of methods, and the research results may provide inspiration for the structural design and optimization of PVDF flexible pressure sensors, to obtain higher sensor sensitivity.