The invention relates to a slope stability influence factor testing device and method and particularly discloses a device for drag force testing by simulating that a slope undergoes a surface runoff effect. The drag force testing device comprises a simulation tank defined by a base plate, side blocking plates, a fixed-end blocking plate and a lifting-end blocking plate and a simulated rock-soil layer laid on the base plate, wherein the simulation tank comprises a fixed end and a lifting end, the fixed end is fixed through a horizontal shaft and can rotate around the horizontal shaft, the lifting end is connected with a lifting device, and the lifting device can continuously adjust the height of the lifting end in the vertical direction; the fixed-end blocking plate is provided with a simulation tank water inlet connected with a water source,, and the lifting-end blocking plate is detachably installed. The invention also discloses a corresponding slope waterflow drag force testing method. The device and the method have the advantages that the device and the method are applicable to waterflow drag force testing of slopes with smaller gradient under the condition of strong rainfall, accordingly the influence of waterflow on slope soil body instability under the condition of strong rainfall, is quantified, and an existing method for quantitative evaluation of slope stability underthe rainfall condition is optimized.