The commonly used forest fire detection methods are insufficient to detect the early- stage forest fire. In order to solve this problem, this paper presents a novel method to detect early-stage forest fire using TDLAS (Tunable Diode Laser Absorption Spectroscopy) technique. With the high sensitivity and rapid response, TDLAS is used to detect trace gas CO concentration sourcing from the early-stage forest fire. The TDLAS system is composed of a White-type multi- pass cell, combined with wavelength modulation and harmonic detection. The analysis of second- harmonic spectrum and the lab tests to different CO concentrations show that the relationship between CO concentration and spectrum intensity abide by linearity relations, and the system has high accuracy. It can be concluded that it is feasible to detect early-stage forest fire by measuring CO concentration and it interests to improve the system performance in the forest fire detection field.