An optical passive imaging interference system (OPIIS) was proposed for the real-time and long-term detection of hydrothermal methane′s concentration, temperature and pressure. Firstly, the forward model that consisted of deep ocean gas emission model, seawater transmission model and instrument responding model was built by interface description language (IDL), and its forward interference fringers were simulated. The SNRs of the forward interference fringes were in the range of (50-70) in general. And the detection sensitivity of concentration measurement is was at least 0.1 mmol/L, the temperature was at least 2 K, and the pressure was at least 0.1 MPa. Then, OPIIS′s data were processed accurately and rapidly by combining imaging interference technology and partial least squares (PLS) algorithm. The multi-dependent variable PLS regression model of methane was established by using 25 modeling samples, and this PLS regression model was cross-validated by using 25 prediction samples. And the max error for concentration prediction of this regression model was 1.9%, for temperature prediction was 0.38%, and 1.0% for pressure prediction.