Surface decarburization of high silicon spring steel in ambient air was studied. The experimental results confirmed the decarburized mechanism under A_C1 temperature, in the temperature range of A_C1-A_C3 and A_C3-G. Under A_C1 temperature, pearlite spheroidization and surface decarburization are carried out simultaneously and pearlite spheroidization is reinforced. Considering the oxidation loss depth, the “true ferrite decarburized depth” at 850 °C (A_C3-G) is still smaller than that at 760°C (A_C1-A_C3). That is because an “incubation period” must pass away before ferrite decarburization occurs in the temperature range of A_C3-G, and the ferrite decarburized rate is limited to being equal to the partial decarburized rate.