The acquisition of quadrature carriers is important for interferometric fiber-optic seismometer with heterodyne scheme. Harmonic distortion and relative amplitude error are the major parameters for quality evaluating. The existence of phase bias between quadrature carriers will lead to the harmonic distortion of demodulation result and increase the relative amplitude error. Relationship among phase bias of quadrature carriers, harmonic distortion and relative amplitude error is analyzed. Mathematical derivation is carried out with series decomposition and Bessel function expansion, and simulation verification is carried out. Analysis results show that the relative amplitude error and the harmonic distortion are both positively correlate with the phase bias of quadrature carriers. The phase bias should be controlled under 1.6×10 -2 rad to ensure relative amplitude error lower than 1% and harmonic distortion better than -40 dB. A experimental system is established, and experiments are conducted. The experimental results are consistent with the theoretical results.