This research focused on the action depth of portable falling weight deflectometer (PFWD) impact load on road structure and proposed reasonable testing parameters for PFWD. Influence parameters of PFWD maximum load were analyzed and stress allowance θ was proposed as an index of stress attenuation. According to the stress solution of elastic half-space under rigid bearing plate, the relationship between θ and depth and radius of single-layer structure was established. Then the finite element model of double-layer structure under rigid bearing plate was built to analyze the effects of thickness, radius, modulus, and Poisson’s ratio on the vertical stress, and the relationship between θ and modulus ratio and thickness was established. The action depth was defined as the depth at which the stress decreases to a certain value. Results show that parameters such as the weight, height of the hammer, and the radius of the plate have effects on the maximum load. The degree of stress attenuation in a single layer structure depends only on the ratio of depth to plate radius. Stress attenuation becomes faster with the increase of the upper layer modulus, while it slows down with the increase of underlying layer modulus. Poisson’s ratio almost has no effect on the stress attenuation. The action depth of PFWD is different with different configurations (hammer weight, plate radius), which is recommended in accordance with analysis result and engineering practice, and reasonable parameters configuration was then proposed based on the stiffness ratio of the tested layer and the underlying layer.