Abstract: During the nighttime, the 135.6nm spectral line is excited by the radiation recombination process of F region O⁺ and e^- and the mutual neutralization process of O⁺ and O^- in the ionosphere. There is a strong correlation between the intensity of the spectral line and the maximum electronic density of ionospheric F2 layer (NmF2). Based on the physical model in which the OI 135.6 nm emission is proportional to the square of NmF2, we establish a retrieval algorithm suitable for different longitude and latitude, local time, season and solar activity. In this paper, the critical frequency of ionospheric F2 region (foF2) was retrieved from 135.6nm emission observed by the Special Sensor Ultraviolet Spectrographic Imager (SSUSI) instrument on board the Defense Meteorological Satellite Program (DMSP), and then the estimated results were compared with the detection results of ground-based ionosonde. As the results show, during the high-solar activity year (2013), the data with relative error less than or equal to 20% accounted for 92.99%, and the average relative error was about 7.18%. During the low-solar activity years (2017), the data with relative error less than or equal to 20% accounted for 80.76%, and the average relative error was about 13.02%. Finally, we analyzed the difference of retrieval accuracy of the algorithm during the high and low solar activity years.