With an atomic beamapparatus and two lasers at λ1 = 852.1nm and λ2=510nm, the Cs atoms are excited to high Rydberg states via a two-step process. The Starkeffect of the Rydberg Cs atom has been experimentally studied in the range of externalfield 0～600 V/cm, the map of the Stark spectra reveal profound “l-mixing” and “n-mixing” of the Stark sublevels.