Tunable diode laser absorption spectroscopy (TDLAS) technology is a kind of fast time response, large-range, continuous on-line monitoring gas detection technique. It is the mainstream technology of gas detection. In this paper the multimode laser diode was used as light source. Multi-mode laser combined with correlation spectroscopy can improve the test reliability and stability. It can also conquer the problem of the central wavelength change of the single mode diode laser due to thermal or mechanical fluctuations in durable working process. A FP laser was used as the light source in this research. A multi-mode diode laser system based on correlation spectroscopy and wavelength modulation spectroscopy (TMDL-COSPEC-WMS) was used to measure carbon dioxide in ambient air around 1 570 nm. The carbon dioxide concentrations were derived from the relationship between the normalized WMS-2f signal peak heights of the measurement and reference signals which selected based on high signal to noise ratio and correlation coefficient. All measurements were performed with controlled carbon dioxide and nitrogen mixtures in which carbon dioxide concentrations range from 0.6% to 30%. The calculation results showed that there was a high linear relationship between the measured and actual carbon dioxide concentration, the linearity was 0.998 7 and the fitted slope was 1.061±0.016 8 respectively over the tested range. A detection limit of 335 ppm·m was achieved. The standard deviation of 0.036 7% was achieved using 20 successive measurements with each measurement time taking ~10 s during 20 minutes, which demonstrated good stability of the system. Good agreements between the measurements of the system and actual values confirm the accuracy and potential utility of the system for carbon dioxide detection.