He was taken to the ICU at Metropolitan Hospital in Dhaka with a brain haemorrhage on Wednesday.
The surgery took place there around 6pm Friday and it was led by neurosurgeon Kanak Kanti Barua of Bangabandhu Sheikh Mujib Medical University (BSMMU).
Barua told bdnews24.com there had been massive haemorrhage in Das's brain and nothing could be said in the next 48 hours.
He added the renowned musician was also suffering from cardiac diseases.
Das's son Pinu Sen Das said his father fell ill at his home in Sylhet on Tuesday. He was admitted to a local hospital first where his brain haemorrhage was detected.
He was brought to Dhaka the following day.
Metropolitan Hospital doctors said Das lost consciousness when the haemorrhage spread on Thursday night.
Ramkanai Das was born in Sunamganj in 1935.
In 2014, the government awarded him 'Ekushey Padak', one of the highest civilian awards of Bangladesh, for his contribution in collecting folk songs.
However, Pandit Ramkanai’s career in art and culture took a hit last year when he was accused of plagiarism and being 'dishonest'.
A section of cultural activists of Sylhet had alleged that Das had taken credit for many popular songs which apparently were compositions of other lyricists.
They had demanded dropping Das's name from the list of Ekushey Padak winners this year.
A Sylhet court in November last year had found Das guilty and ordered a stop to the publication of his book 'Sangeet o Amar Jibon' (Music and My Life) in a case filed by late folk song artiste Bidit Lal Das over similar allegations.