The game was slow to start with both kickers exchanging early penalties, but tries from Henry Taylor and Ben Botica in the first 40 minutes, gave Harlequins a healthy 17-3 lead at the break.
With a yellow card to Welsh's Billy Moss early in the second half it was the home side, playing at the away side's venue who capitalised the most, out matching Quins' scoring by two tries to one during the sin bin period.
Harlequins quickly overcame the Welsh's dominance and scored their bonus point try from Sam Smith. Seb Stegmann took a brace of tries, before Welsh's James Ngwa completed the night's scoring.
Ben Botica again impressed as his points total of 21 consisted of one try and 16 points from the boot.
After both kickers exchanged penalties, Harlequins began to assert some dominance. Strong running from Sam Smith and Rob Buchanan was a sign of positive things to come.
Ben Botica opened the try scoring on 19 minutes. The real credit was due to Rory Clegg. From a lineout in midfield, the fly half carved through the on rushing Welsh defence, Clegg's decision making while running down the middle channel, sucked in the defence. This gave Botica an easy score under the posts and the centre convert his own score as Quins lead 10-3.
Stegmann came close moments later when he charged down deep in the 22. Again the accuracy of Clegg was pivotal. The fly-half grubber kicked through the defence which found Ashley Smith five metres from his line. Taking too long with the kick, Stegmann pounced and was unlucky when the ball rolled dead before he could touch down.
Such was the game that Clegg was having, luck was on his side. When a penalty kick looked unlikely to go to touch, a mistake by theWelsh back three allowed the ball to bounce out five metres from the line. This would ultimately lead to a half time Quins score.
From the lineout, which was consistently good all evening, a rolling maul formed with Darryl Marfo and Buchanan controlling it at the back well. When the ball went to ground, Henry Taylor sniped around the side of the ruck to score. Botica again converted for a 17-3 lead at the break.
Moss' was sin-binned on 43 minutes for repeated infringements and Harlequins made their numerical advantage count immediately. From the scrum, Taylor threw a wide pass which found Stegmann to score. Botica again made light work of the touchline conversion, extending Quins' lead to 24-3.
Despite being one man down, Welsh clawed back two tries in quick succession. The first, scored by Ashley Smith, resulted after Welsh worked the phases close to the Quins line on 53 minutes, the winger took a wide pass and bundled in at the corner
Miss communication in the Halrequins back line led allowed Ryan Davies to pick up a loose pass. As he charged down field, scrum-half Jonah Holmes, who had moved to the wing received the offload to score. Davies added both conversions
Harlequins struck back again with their bonus point score from Sam Smith. The try came as a result of stunning breaks up field by Ollie Lindsay-Hague and captain, Luke Wallace. As the replacement fullback and flanker darted around tackles, this took Quins into the 22. Sam Smith received the ball and dived over. Botica converted giving Quins a 31-17
It was then the turn of Quins to score a run away try from mistake in Welsh's midfield. As the ball was spun wide, Stegmann scooped a loose pass up and ran 60 metres to score. Botica Converted to end Harlequins' scoring.
James Ngwa scored the final try in 78th minute, Davies conversion hit the post to complete the evening's scoring.
