A detection circuit is electrically connected with a static transfer switch. The static transfer switch includes a silicon controlled rectifier. The detection circuit includes a high-pass filter, a low-pass filter, an absolute value circuit and a determination unit. After the high-pass filter filters off a low-frequency component of a terminal voltage between the first terminal and the second terminal of the static transfer switch, a first signal is generated. After a high-frequency component of the first signal is filtered off by the low-pass filter, a second signal is generated. The second signal is converted into an absolute value signal by the absolute value circuit. If no pulse signals are contained in the absolute value signal, the determination unit determines that the static transfer switch is in an abnormal on condition.