Abstract: With the development of the ultraviolet spectral observation technique and the needs of the space weather research, the far ultraviolet remote sensing becomes one of the most important approach for the upper atmosphere and ionosphere diagnosis. This paper discusses how to retrieve the ionospheric electron density profile (EDP) from the far ultraviolet limb-viewing spectrographic imager, operating at OI 135.6 nm. Firstly, the radiation mechanism of OI 135.6 nm nightglow is presented. Then, a discrete limb-observation model is built based on the assumption that the ionosphere is spherically symmetric and has a layered structure. The OI 135.6 nm volume emission rate as a function of the discrete altitude can be inferred by using the non-negative least squares method, then the discrete EDP is determined by taking the square root of the volume emission rate. In the end, the discrete EDP is deduced from the OI 135.6 nm limb imaging data of the Global Ultraviolet Imager aboard TIMED satellite, and the distributions of the retrieved results along the altitude and the geographic longitude agree very well with those of the EDP provided by the GUVI data product.