He sustained the injuries after falling about three or four floors in the early hours of yesterday morning.
It is believed that the man had been spraying graffiti at Westmoreland House building at the junction of Stokes Croft and Ashley Road.
He is described as being in a "very serious" condition and is expected to remain in hospital for some time.
Chief Superintendent John Long of Avon and Somerset police said: "Police were recently criticised for deciding not to access the building to arrest two graffiti suspects.
"Officers are not routinely equipped to scale heights and operate on hazardous sites and it would only put officers and others in danger by attempting to do so in those circumstances.
"The unfortunate events yesterday where a man sustained serious injuries after falling within the building reiterates the danger it poses and the initial concerns."
He added: "Police will continue to work with its partners to rid the city of the graffiti which can have a detrimental effect on communities and affect the quality of life of local residents."
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