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.