With the conjunction of tunable diode laser absorption spectroscopy technology(TDLAS) and the open long optical path technology,the system designing scheme of CO 2 on-line monitoring based on near infrared tunable diode laser absorption spectroscopy technology was discussed in detail,and the instrument for large-range measurement was set up.By choosing the infrared absorption line of CO 2 at 1.57 μm whose line strength is strong and suitable for measurement,the ambient atmospheric CO 2 was measured continuously with a 30 s temporal resolution at an suburb site in the autumn of 2007.The diurnal atmospheric variations of CO 2 and continuous monitoring results were presented.The results show that the variation in CO 2 concentration has an obvious diurnal periodicity in suburb where the air is free of interference and contamination.The general characteristic of diurnal variation is that the concentration is low in the daytime and high at night,so it matches the photosynthesis trend.The instrument can detect gas concentration online with high resolution,high sensitivity,high precision,short response time and many other advantages,the monitoring requires no gas sampling,the calibration is easy,and the detection limit is about 4.2×10^-7.It has been proved that the system and measurement project are feasible,so it is an effective method for gas flux continuous online monitoring of large range in ecosystem based on TDLAS technology.