Abstract: Compound-specific isotope analysis (CSIA) online is an indispensable key technique for identifying the sources of organic contaminants in the environment and characterizing their transformation processes, but there are still some problems in practical applications. In this study, firstly, the development of six methods for online determination of compound-specific organic isotope analysis was introduced and reviewed, including Gas Chromatography-Isotope Ratio Mass Spectrometer (GC-IRMS), Liquid Chromatography Coupled-Isotope Ratio Mass Spectrometer (LC-IRMS), Direct Introduction-Gas Chromatography-Isotope Ratio Mass Spectrometer (DI-GC-IRMS), Gas Chromatography-Quadrupole Mass Spectrometer (GC-qMS), Gas Chromatography-Multicollector Inductively Coupled Plasma-Mass Spectrograph (GC-MC-ICPMS) and Gas Chromatography-Cavity Ring-Down Spectroscopy (GC-CRDS). Secondly, five noteworthy problems in online testing were proposed: sample pre-concentration, Gas Chromatography (GC) and Liquid Chromatography (LC) separation, selection of instruments and methods, the development of organic compound stable isotope standard material and safety control. Finally, three suggestions were proposed: 1) to develop vigorously the direct injection technology without combustion, such as GC-qMS and GC-CRDS, 2) to continually develop the determination technology for organic chlorine and organic bromine isotopes and 3) to rapidly develop international stable isotope standards for organic compounds. The new understanding in this study was that when multi-element isotope analysis was applied to study compound-specific isotopes, direct injection technology without combustion was the optimal choice.