Reverberation time is a major factor which affects the auditory environment in the classroom. Solution and shape of the classroom can be determined by simulation software, which meets the legislation of reverberation time. Simulation can be made by SABINE, NOR-ER and MIL-SE reverberation calculation formula under different conditions. With the condition of empty, 2/3 and full occupation of a medium classroom, reverberation time was calculated by these three formulas. Results of calculation were compared with former experimental data. Difference and applicability of reverberation formulas are analyzed. The auditory environment of classroom can be improved by such results at the design solution stage.