The 32-year-old fell on to the line at Bridge Street Station shortly before noon on Thursday.
He was taken to the city's Queen Elizabeth University Hospital for treatment.
British Transport Police said officers were called to the station following reports of a casualty on the tracks.
In a statement, the force said: "Paramedics also attended, and a 32-year-old man has been taken to hospital.
"At this stage his injuries are believed to be life-threatening. The incident is not being treated as suspicious."
The man is reported to have been with his guide dog at the time, which was taken into the care of the Scottish SPCA.
Glasgow Subway tweeted both the inner and outer circles were suspended because of a "passenger-related incident", but they both later reopened.
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