This paper described saturation method for determining absorption cross sections by using a setup composed of the pulsed dye lasers system synchronously pumped by a frequency doubled Nd:YAG-Laser, e-type electron gun, apparatus for atomic beam of U, and by using the technique measuring resonant photoionization spectrum and time-of-flight mass spectrum at the same time. This method has very strong selectivity, extremely high sensitivity and resolving power, which is developed on the base of single-atom detection technology.