US Weekly reports that the singer recently hired celebrity fitness expert Tracy Anderson to help her shed pounds before the big event via an intense workout schedule.
"She wants to look healthy," Anderson told the publication.
Simpson is said to be partaking in 75-minute long workouts four times per week, which include dance aerobics and strength training.
A source added: "[Jessica]'s seeing results. She really loves it. She thinks it's so much fun."
Simpson recently confessed that she was "shocked' by Johnson's marriage proposal.