Abstract: When fluid seepage enters the pore throat of reservoir rocks, it exerts a seepage force on the rock matrix, breaking the original stress balance state of the reservoir rock and affecting its deformation and failure. Although a large number of experiments and numerical simulation studies have confirmed the significant impact of seepage forces on rock failure, research on seepage forces in the field of petroleum engineering is scarce. The mechanism of seepage force on the initiation and expansion of hydraulic fracturing cracks remains unclear. Based on this, this paper first studied the mechanism of seepage force when fracturing fluid seeps into the rock pores, using the definition of geomechanical seepage force and Biot consolidation theory. Then, taking an open hole as an example, we analyzed the stress field formed by the seepage force, derived a formation breakdown pressure analytical solution formula considering the effect of seepage force, and revealed the influence of seepage force on the formation breakdown pressure of the open hole. The results show that when the pore pressure difference is equal to one atmospheric pressure, the volumetric seepage force per cubic centimeter of rock sample greatly exceeds the gravitational force on rock samples of the same size. Therefore, the effect of seepage force on reservoir rocks is significant and cannot be ignored. When fracturing fluid seeps into the pore throats of reservoir rocks, it exerts a seepage force on the rock matrix, which can significantly reduce the effective circumferential stress around the open hole and increase the likelihood of tensile failure on the well wall. The greater the Biot effective stress coefficient, the stronger the effect of seepage force, and the wider the range of the stress field around the wellbore affected by seepage force. Compared to impermeable reservoirs, seepage force substantially reduces the formation breakdown pressure of open hole. The impact of seepage force on the formation breakdown pressure of unconventional reservoirs with deeper depths and smaller bi-directional stress differences is more significant.