The study of gas spectrum characteristics at high pressure is the foundation for tunable diode absorption spectroscopy technique used in pulsed detonation engines and other high-pressure combustion environments. To understand variations of gas spectrum characteristics with pressure, especially the absorption spectrum characteristics at high pressures, gas absorption spectrum characteristics at high pressure are studied by tunable diode laser absorption spectroscopy in this paper, a method to calculate gas concentration at high pressure is also presented. Within a pressure range of 1-10.13×105 Pa, CO2 absorption spectrum in a near-infrared band of 1.58 μm is simulated. Gas online measurement at high pressure is set up. The CO2 absorption spectrum is measured by a direct absorption method near 1578.0-1579.7 nm. The concentration at high-pressure environment is calculated by linear fitting method and compared with theoretical absorption spectrum. The results show that the absorption spectrum obtained experimentally is in good agreement with simulations result. In high-pressure environment, maximum error of linear fitting method to calculate gas concentration at high pressure is 5.5%, and the average error is 2.6%.