The second-order coherence of radiation field in Λ-type three-energy level electromagnetically induced transparency (EIT) system is investigated theoretically. The result of calculation indicates that the coherence effect of atom in EIT system induces the second-order coherence of the resonance fluorescence field of upper level to present the quantum statistical property of single photon. And the second-order coherence of the radiation field dependence of intensity of coupling light and detuning of probe light is considered. Under the condition of Ω>(Γ2+Γ3)/2, if the weaker power of coupling light or far-off-resonance of probe light is applied, second-order coherence can retain smaller than 1 in longer time delay, which can make for realizing the quantum statistical action of nonclassical field; contrarily, under the condition of Ω≤(Γ2+Γ3)/2, the smaller off-resonance of probe light is applied, the better the quantum statistical nonclassical field realizes that second-order coherence can retain smaller than 1 in longer time delay. So the quantum statistical property of single-photon field in EIT can be optimized in different conditions when appropriate parameters are chosen.