We report a new method for detecting highly excited ions. With a strong constant electric field ionization following the extracting of ions by a pulsed electric field in a time-of-flight mass spectrometer, the direct multi-photon ionization background can be eliminated in laser multi-step resonance ionization mass spectroscopy of ionic Rydberg states. A high detection efficiency can be obtained for low members of ionic Rydberg states by use of this method. We have applied the method in the study of the spectroscopy of ionic Rydberg states in ytterbium.