A method to three-dimensional position moving particles with one lens and two cameras is proposed. Two particle images with different degrees of defocusing are adopted to solve the ambiguous problem of particle positions. A single-lens dual-camera system is developed to simultaneously capture these two images for the moving particles. The measurement principles and theoretical analysis are introduced first, and then simulated investigations and experimental research are discussed. The measurement errors in the simulations and experiments are less than 1% and 4%, respectively, in 20 times the depth of field of the system, which validates the feasibility of this method.