Roads policing officers who stopped the man said he could only see a small fraction of the road every time he glanced at his laptop screen.
Supt Andrew Reeves said the trucker, stopped near Maidstone, was fined Â£300.
He said any momentary lapse in concentration, such as looking at a device, could have disastrous and life-changing consequences.
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The driver was one of 37 stopped and penalised for using mobile devices, speeding and other offences in a five-day crackdown on the motorway.
Kent Police said one sports car was stopped for driving at 105mph on the coastbound carriageway.
Officers patrolled the route in an unmarked 40-tonne lorry, which allows them to see into cars and larger vehicles.
Police tackling dangerous driving have stopped and fined a lorry driver for watching a movie on the M20.