Development of a two color interferometer on a field-reversed configuration device

•The initial density measurement results of the HFRC theta-pinch plasma formation region are presented for the first time.•The interferometer has a time resolution of 40 MHz by using the acousto-optic modulator (AOM), which can benefit for the future physical research such as observation of the rotational instabilities on HFRC.•The interferometer eliminates the phase error caused by mechanical vibration and fringe jump during measurement by adopting CO2 laser and HeNe laser as the source. The experimental results show that the phase noise of the two color interferometer system is within 1.7° and the residual error is less than 1 × 1018 m−2.