An antenna testing device includes an analyzer, a transmission probe electrically connected to the analyzer, a receiving probe electrically connected to the analyzer, and a shielded box having a cutoff frequency. The analyzer generates a test signal the frequency of which is lower than the cutoff frequency, the transmission probe receive the test signal and sends the test signal to the shielded box. The antenna is coupled with the transmission probe and generates a coupled signal, the receiving probe receives the coupled signal and sends the coupled signal to the analyzer. The analyzer analyzes the coupled signal and the test signal, the analyzer calculates the return loss of the antenna.