The real-time laser frequency scale system is developed for measuring the carbon isotopic abundance. By using confocal Fabry-Perot interferometer with different free spectral ranges, the diode laser frequency tuning characteristic is acquired in each laser frequency scanning period. During the course of measurements, two nonlinear analysis techniques are tested, i.e., linear interpolation and polynomial fitting. The absorption line center position for CO2 between 4976 and 4980 cm-1 is obtained by 100 average measurements from our experiments. A comparison between the measured absorption spectra and the spectral line positions from the HITRAN-2008 database, shows that the two method can both reach an accuracy of 10-4 cm-1 for the laser frequency scale. The best results are obtained with a linear interpolation method. The result shows that the laser frequency real-time scale system is applicable in carbon isotopic abundances measurement.