As one of the most important greenhouse gases, CO₂ has attracted much attention for its change and distribution. The differential absorption laser radar (DIAL) system is an effective means to detect the concentration of CO₂, which is of great significance for studying the source and sink of greenhouse gas. This paper mainly studies the detection of CO₂ average concentration by sinusoidal modulated continuous wave (cw) DIAL on the horizontal path. Using the absorption spectrum of CO₂ and H₂O in HITRAN database, considering the absorption cross section of CO₂ and the interference of H₂O, the detection wavelength of DIAL is selected as: On-line is 1 572.335 nm and Off-line is 1 572.180 nm; Acousto-optic modulator replaces electro-optic modulator to transmit cw laser and the frequency of the two modulation signals is slightly different. The On-line modulation frequency is 101.833 kHz, and the Off-line modulation frequency is 99.733 kHz. On-line is locked at the frequency of absorption peak of the gas pool at 1 572.335 nm by spectral modulation technology, and the residual amplitude modulation (RAM) of the phase modulator is eliminated by applying DC bias feedback voltage on the phase modulator in the research process. The system achieves a continuous output wavelength RMS error of 0.05 pm at 12 hours. Fast Fourier transform (FFT) is used to acquire the power spectrum of the echo signal and the laser intensity at the monitor in the CPU. The ranging of the target is possible by utilizing the phase difference between the monitored and the received signals; The optical circuit is fiber based, and this makes the system compact and reliable. The field experiment and comparative experiment were carried out to obtain the average concentration of CO₂ on the 1.3 km path in Shanghai. The experimental data showed that the system observation accuracy was 4 ppm(ppm is parts per million), and the detected daily variation trend coincides with the in situ CO₂ sensors (LI7500A).