The invention discloses a stable isotopic abundance ratio real-time online monitoring device and a stable isotopic abundance ratio real-time online monitoring method. A wavelength scanning off-axis integrated cavity spectrum technology is utilized for performing precision measurement on isotopic abundance ratio of an object element, and the device comprises a laser device, a laser controller, a signal generator, an angle adjustable prism, a wavemeter, an off-axis integrated cavity, a temperature and pressure controller, a piezoelectric ceramic controller, an off-axis parabolic mirror, a photoelectric detector, a lock-in amplifier and a signal processing system for acquiring A/D (analog/digital) and terminal signals. The method disclosed by the invention is performed on the basis of the off-axis integrated cavity laser absorption spectrum technology and is further combined with wavelength scanning, cavity regulation, temperature, pressure and wavelength precision control technology, so that the stable isotopic abundance ratio real-time online monitoring can be realized, and trace gas concentration and isotopic abundance ratio measurement can be simultaneously performed; furthermore, the preparation of a sample is not required, so that the measurement frequency is greatly improved; and the device has the advantages of simplicity in operation, no need of calibration, almost no drift during a long period of time, good stability, low power, convenience in carrying, mounting and field experiments and large dynamic range.