It was a fourth successive victory for the Red Badgers with five different scorers and a 20th of the season for Tyler Blake - the first player since Patrick Webb in 2019-20 to score 20 goals from open play for the club in a single season.
With William Robinson making his first full start for Eastwood, the home side had a frustrating first half hour with mid-table Harrowby starting off the better side.
However, that all changed once the breakthrough came on 37 minutes via a composed finish by Terry Hawkridge from the edge of the box.
Just two minutes later a great ball from Aaron O'Connnor found Declan Dunn at the back post to head home a second.
Eastwood keeper Sam Wilson did have to make a late save as Eastwood went in 2-0 up.
But it was 3-0 on 61 minutes as Hawkridge sent the ball over the top to find Blake, who outmuscled a defender and finished really well.
Six minutes later Aaron Opoku fed O’Connor, who finished with class, rolling the ball between the defender's legs past the goalkeeper.
But within 60 seconds Eastwood were caught napping as Harrowby pulled one back through Harry Allcock to spoil the clean sheet.
Defender Phil Watt then completed the rout with 10 minutes remaining as he headed home from a Hawridge corner – his fifth of the season.
The Red Badgers now host Melton Town on Saturday, who have improved over recent games after seeing a dreadful run drop them down the table from an early promotion challenge.
Neighbours Kimberley MW also had a fine day on Saturday as they recorded their first away win of a testing campaign with a 1-0 success at Bourne Town.
It moved them out of the bottom two for the first time since September with a great chance to build on that with a trip to relegation rivals Deeping Rangers this weekend.
Kamani Wilson was Kimberley's matchwinner, scoring three minutes before half-time.
But it was a frustrating day for Selston in Division One as their home clash with Dunkirk was called off due to a waterlogged pitch after a morning inspection.