Binary frequency shift keying（2FSK） is widely used in the power line carrier communication systems， however the traditional modulation and demodulation of 2FSK are often implemented using real signal processing. A novel method of digital complex modulation and complex demodulation based on complex signal processing to implement 2FSK is proposed. According to the bit value of the input data， the complex low-pass signal is produced and the signal is then low-pass filtered and complex modulated to finish complex modulation. The input signal of 2FSK is sampled， then complex demodulated and low-pass filtered to obtain complex low-pass signal. The complex low-pass signal and the conjugate signal of its delayed version are multiplied. According to the image part of the product， the complex demodulation is finished.