We have obtained Rydberg atom of different |ml| value by two-step excitation using polarized lasers. The field-ionization thresholds for 5snd (n= 22-27), | ml| =0,1,2 Rydberg states of Sr atom is measured by using a pulse electric field (up to 2.4kV/cm). Second-order approximate formula of field-ionization threshold that depend on the |ml| value is obtained and compared with the measured data.