A system of integrated cavity output spectroscopy (ICOS) based on a tunable distributed feedback (DFB) diode laser (TDL) is described. In this system a tunable DFB diode laser with the central output wavelength 1315 nm is used as the light source, the length of the absorption cell is 38 cm which is consisted by two mirrors with high reflectivity, near the laser central wavelength 1315 nm, the mirror′s reflectivity is about 99.7%. Presentation of experimental principle, experimental scheme and diode laser wavelength calibration are given; As experimental result, the absorption spectrum of water vapour at 7612.026, 7612.269 and 7610.224 cm-1 are obtained and a detection sensitivity of 2.02×10-7 cm-1 has been achieved. It is experimentally demonstrated that ICOS is a high sensitive and high resolution spectral technology with the advantage of simple experimental setup and easy operation.