The invention provides a circuit test system and a circuit test method. The circuit test system includes a circuit test apparatus and a circuit to be tested. The circuit test apparatus provides a first clock signal. The circuit to be tested includes a plurality of input/output pads and at least one clock pad. At least two input/output pads of the input/output pads are connected to each other to form a test loop during a test mode. The clock pad receives the first clock signal. The circuit to be tested multiplies a frequency of the first clock signal to generate a second clock signal, and the test loop of the circuit to be tested is tested based on the second clock signal during the test mode. The frequency of the second clock signal is higher than that of the first clock signal. Furthermore, a circuit test method of the foregoing circuit test system is also provided.