The invention discloses a diagnostic circuit and diagnostic method for a vehicle AC charging device, the diagnostic circuit comprises a detection circuit, a state judgment circuit and a relay drive circuit; the detection circuit comprises a first half-wave rectification circuit and a second half-wave rectification circuit; the relay drive circuit comprises a first relay input terminal, a second relay input terminal, a first relay and a second relay; the state judgment circuit comprises a control terminal, the state judgment circuit controls a negative input terminal of the first half-wave rectification circuit and on-off of the first relay input terminal according to a signal of the control terminal, or the state judgment circuit controls a negative input terminal of the second half-wave rectification circuit and on-off of the second relay input terminal according to a signal of the control terminal. According to the invention, the circuit and the method can realize time-sharing detection for relay state and power supply input frequency.