Mechanical force has been found to be an alternative way to non-electrically switch the polarization of ultrathin ferroelectric films owing to the flexoelectric effect. Reducing the required force for switching is desirable for a low risk of damage to both sample and tip. Here, the strain dependence of mechanical threshold force has been studied in ultrathin PbZr0.2Ti0.8O3 films. The mechanical threshold force for polarization reversal reduces remarkably by a factor of ∼5 with decreasing the compressive strain, associated with a reduction of coercivity and tetragonality. We attributed such behavior to the reduction of switching barrier and remnant polarization. Our work provides a route to realize ultra-low mechanical writing force for non-volatile memory applications.