The ionization spectrum of potassium atom is investigated by 4 2S 1/2 6 2S 1/2 two photon resonant excitation. The resonant exciting laser wavelength is 728.4 nm. The ionization spectrum as a function of temperature and laser intensity are discussed. When the temperature of potassium vapor comes to 643 K, the ionization intensities reach the maximum. And the ionization intensities are saturated while laser intensity increased to 4.5×105 W/mm2. The spontaneous spectrum of the 6 2S 1/2 4 2P 1/2,3/2 is also observed.