Imidazolium-based ionic liquid functionalized salen Mn complex (IL-salen Mn) was synthesized and employed as the only catalyst for the one-pot transformation of styrene and CO2 into styrene carbonate. First, styrene was converted efficiently into styrene oxide by using urea hydrogen peroxide (UHP) as an oxide in the presence of pyridine N-oxide (PyNO). Subsequently, the as-obtained styrene oxide was transformed into styrene carbonate via cycloaddition with CO2. The effects of reaction parameters such as catalyst type and dosage, additive amount, oxide type and amount, reaction temperature, reaction time, and pressure of CO2 were studied detailedly. When 8 mol% (based on the total mol of reactant) IL-salen Mn was used as catalyst, the molar ratio of styrene, UHP and PyNO was 1:3:0.2, epoxidation temperature was 30 ℃, epoxidation time was 5 h, the cycloaddition temperature was 80 ℃, cycloaddition time was 12 h, CO2 pressure was 1 MPa, the conversion of styrene (90 %) and selectivity of cyclic carbonate (32 %) were obtained. Furthermore, according to previous studies and in combination with the kinetic results, we proposed the possible mechanism for the tandem reaction.