The 20-year-old joined the League Two outfit at the beginning of January, but he had been expected to return to Loftus Road at the end of the month.
However, Harriman has revealed that he has agreed to extend his stay with Gareth Ainsworth's side, telling the club's official website: "I'm very happy to be staying for the extra few months.
"I'm playing regularly and really enjoying it. Playing for Gaz is great and the lads have been really welcoming - they're a great bunch."
Harriman has made four appearances for Wycombe this month.