we demonstrated a high sensitive trap-loss spectroscopy technique by modulating fluorescence of cold atoms in magneto-optical trap, which allow a direct spectroscopy detection of the rovibrational levels with a very weak transition probability. The low-lying vibrational spectroscopy of upsilon = 3 approximately 17 of Cs(2) 0g- pure long-range state have been observed with rotational structures, which are well resolved up to J = 8. The rotational constants are obtained by fitting experimental data to a nonrigid rotation model.