The 19-year-old netted on her debut for Arsenal Ladies in 2013 and scored 22 goals in her first full season with the north London club.
Humphrey is the Belles sixth signing following their promotion to Women's Super League One.
"I want to get some more experience and help the team do as well as we can," she told the club website.

The note was sent in 1960 to the family of Stan Laurel's childhood friend Margaret Miller, who went on to marry the star's school friend.
The signed and dated letter thanks Mrs Miller for her belated Christmas card calendar and talks about his "many happy memories" of Tynemouth Pavilion.
Newcastle auctioneer Anderson and Garland said the item could create a "bit of a frenzy" among bidders.
Mrs Miller's daughter, also called Margaret, found the letter in her brother's belongings when she moved house.
The 88-year-old from Hexham, said: "My father and Stan Laurel were at school together in Tynemouth in the 1890s.
"Stan's family lived in North Shields and my father's family knew them quite well.
"My parents were going to the United States on holiday and hoped to meet up with him.
"I don't think they did and I imagine that's why my mother sent the calendar to him because he didn't get to see it."
Laurel was born in Ulverston and later emigrated to the US where he found fame in Hollywood.
Before emigrating, he spent much of his life in the north-east of England, going to school in Bishop Auckland, County Durham, and Tynemouth.
Anderson and Garland auctioneer Fred Wyrley-Birch said: "There is such a lovely story behind this letter, which is likely to add to the value of the piece when it goes under the hammer.
"We expect people interested in Stan Laurel's North East connections, as well as autograph collectors, to be among the bidders. It may well create a bit of a frenzy."
It goes to auction on Tuesday and is expected to fetch between Â£300 and Â£500.
Doncaster Rovers Belles have signed Arsenal Ladies' teenage midfielder Carla Humphrey on loan.
[NEXT_CONCEPT]
A letter from Cumbrian-born comedian Stan Laurel is to be auctioned.