In Zero-IF receiver, the radio quadrature demodulator has inherent I/Q gain and phase imbalance. In order to reduce its impact on the demodulation performance, a novel FM discriminator is proposed. The algorithm can track and compensate the I/Q imbalance correctly without modeling, training and parameter-calculating. Without additional hardware cost, the proposed discriminator can be easily implemented. Simulation proves that the proposed discriminator can get same demodulation performance both with I/Q imbalance and ideal quadrature demodulator. Besides, hardware experiments are achieved to demonstrate its effectiveness and reliability.