The invention discloses a new balance detection bias control method for an MZ external modulator. The method comprises the following steps: sending RF (radio frequency) signals and pilot signals into a signal end of the MZ external modulator after synthesizing the RF signals and the pilot signals by a signal synthesizer, wherein modulated light signals pass through an optical splitter and part ofthe optical signals enter an photoelectric detector; converting the part of the optical signals into electric signals by the photoelectric detector; detecting the electric signals; detecting fundamental waves and secondary harmonics of the pilot signals or two or more harmonic components; performing division operation on a transmission function of the two harmonic components and obtaining an adjustment amount of a direct current bias according to a ratio function after the division operation; and obtaining a bias voltage according to the adjustment amount of the direct current bias and sending the bias voltage into the direct current bias end of the MZ external modulator to realize the closed-loop control of an optimal direct current bias point of the MZ external modulator. By using the method, the influence on the precision of a direct current bias control method caused by the output power of a laser and the loss of a link can be eliminated, so that the precision of a system is improved.