The invention discloses a method for determining a phase state of an oil-gas reservoir. The method comprises step S1 of utilizing a comprehensive two-dimensional gas chromatography - time-of-flight mass spectrometer GC x GC- TOFMS to recognize and identity ethyl Noradamantane series compounds in a plurality of crude oil samples, thereby obtaining peak information of each compound; step S2 of quantitatively analyzing the ethyl Noradamantane series compounds to obtain contents of the ethyl Noradamantane series compounds; step S3 of fitting a relation between the content of the ethyl Noradamantane series compounds and gas-oil ratio of an oil reservoir; step S4 of establishing a relationship between the gas-oil ratio of the oil reservoir and the phase state of the oil-gas reservoir; and step S5 of determining the phase state of the oil-gas reservoir. The invention determines the phase state of the oil-gas reservoir by utilizing content of ethyl Noradamantane , thereby providing a new indexfor determining the phase state of a deep-level oil-gas reservoir, and providing a new pattern of solution to oil-gas resource evaluation.