Sammy Quayle fired a double, with Charley Boswell, Daisy McLachlan, Rachel Panting and Emma-Jane May also on target for the Blues.
Pompey finished fifth in the table and Sadler saluted the progress his troops made over the campaign.
The boss said: ‘We’ve made massive progress over the course of the season.
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‘I’m really proud of the way the girls handled the occasion and it was superb to watch at times.
‘We performed really well against QPR, although we were better in the second half than the first, getting the ball down to play some great football.
‘There were a load of opportunities and we could have scored even more, while it was disappointing that we couldn’t quite keep a clean sheet.
‘But I’m really proud of the way that the girls handled the occasion and it was superb to watch at times.’
QPR made a bright start and hit the woodwork in the early stages.
Quayle and Panting had decent chances for the hosts before the deadlock was broken on 25 minutes when captain Boswell fired in her effort from just inside the box.
Pompey doubled their advantage in the 38th minute.
Quayle nicked the ball off of a QPR defender, rounded the goalkeeper and slotted into the empty net.
Less than a minute later, the Blues had their third when Quayle’s reverse pass behind the visitors’ defence was met by McLachlan who fired into the bottom left-hand corner.
On 69 minutes Shannon Albuery was brought down inside the QPR box and Sadler’s side were awarded a penalty.
Panting took the responsibility and stuck her spot-kick home.
Quayle notched her second and Pompey’s fifth of the game in the 81st minute.
And a minute from full-time, Albuery again won the Blues a penalty.
This time it was May who stepped up and slotted into the bottom left-hand corner in front of the Fratton End.
QPR grabbed a consolation goal in the dying embers of the game when Chontele Lawrence curled a fine effort into Levett’s net.
Nevertheless, it was a fine win for Sadler’s side in front of 397 fans in the Fratton End.
– OLLIE COOK