A test system includes a transmitter, capacitor, and receiver. The transmitter includes: an output circuit coupled to the receiver via the capacitor; a transmitting circuit transmitting a predetermined signal to a pad of the output circuit during a first test; a signal generator outputting a first signal and second signal to the pad during a first process and second process of a second test; and a comparator comparing the pad's signal with a reference signal in the first process and second process to determine whether the second test passes. The receiver includes: an input circuit coupled to the transmitter via the capacitor; a switch coupled between the input circuit and a receiving circuit to be conducting in the first test and second process and nonconducting in the first process; and the receiving circuit determining whether the first test passes according to the predetermined signal and assisting the capacitor in discharging.