The invention discloses a system adopting square wave to modulate and measure eigen frequency and half-wave voltage of an interferometric fiber-optic gyroscope, wherein a central processor (63) is composed of a demodulation unit (631), a frequency modulation unit (632), an amplitude modulation unit (633) and a direct digital synthesis unit (634); and the demodulation unit (631) is used to (A) extract a demodulation value P(Delta t<+>) of an effective modulation positive half-period Delta t<+>> from received optical power information V1 during a modulation period Delta T, (B) extract a demodulation value P(Delta t<->) of an effective modulation positive half-period Delta t<-> from received optical power information V1 during a modulation period Delta T, (C) average over the demodulation value P(Delta t<+>) of the positive half-period and the demodulation value P(Delta t<->) of the negative half-period to get demodulation average value P(Delta t), and transmit the demodulation average value P(Delta t) to the amplitude modulation unit (633), (D) extract an optical power average value S (Delta t) from received optical power information V1 during a modulation period Delta T and transmit the optical power average value S (Delta t) to the frequency modulation unit (632). The satisfying condition of the modulation frequency fd is 2fe / 5<Fd<8fe / 5, and the satisfying condition of the initial amplitude A0 is Pi / 8<A0< Pi / 2, and the wide range is enough for application in construction.