Electronic states of the CH3 Se have been studied using the complete active space self-consistent field ( CASSCF)and multiconfiguration second-order perturbation theory( CASPT2 )methods in conjunction with ANO-RCC( TZP)basis set. To investigate the Jahn-Teller effect on the CH3 Se radical,Cs symmetry was used for CH3 Se in calculations. The results showed that the Jahn-Teller effect is very small(69 cm-1 )and the 12 A'state is slightly more stable than 12A"(8 cm-1). The computed C—Se stretching ν6(a')frequency is 554. 1 cm-1 which is in good agreement with the experimental values of 600 ± 60 cm-1 . The calculations of CH3 Se at 3. 621 and 5. 307 eV are attribute to the 12 A'→22 A'(12 A1 )and 12 A'→22 A",respectively.%通过系统的使用完全活化空间自洽场（ CASSCF）和多组态二阶微扰理论（ CASPT2）方法结合ANO-RCC（ TZP）基组研究了CH3 Se自由基的低能电子态相关性质，计算过程中，采用Cs 对称性.实验结果表明，该自由基的Jahn-Teller效应不明显（69 cm-1），基态电子态12 A＇与其简并态12 A＂形成8 cm-1的微小劈裂，这一点与实验值形成较好的吻合.计算所得基态电子态的C—Se的拉伸频率ν6（a＇）为554.1 cm-1与实验值的600±60 cm-1吻合良好.另外计算中发现CH3 Se在0.025 eV、3.661 eV和7.322 eV处存在12 A＇→12 A＂、12 A＇→22 A＇（12 A1）和12 A＇→12 A＂三处比较强烈的激发光谱.
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