Using horizontal three-layer media as the model, this paper deals emphatically with the concealed layer and layer of inverted velocity encountered commonly in the study of first arrival refraction method: their formation condition and their influence on the precision of inversion interpretation. With the applications in Gezhou Dam and Shenzhen Airport as examples, this paper demonstrates that the utilization of the layer velocity and thickness in the survey area to the theoretical calculation helps to the quantitative judgment as to the feasibility of using intersection point method to evaluate effective velocity and the reliability of inversion interpretation of outcomes.