Ultracold 70S Cs Rydberg atoms are obtained by two-photon excitation in a magneto-optical trap, and detected by using the state-selective pulse field ionization technique. The evolution of 70S Rydberg atoms is investigated by changing the delay time between excitation laser and ionization electric field and the excitation duration. Blackbody radiation is taken into account to explain experimental result. The experimental result is found to be consistent with theoretical value.