We don’t learn her true identity until the last act of the film, but why does Padme pretend to be a handmaiden?
Padme chooses to pretend to be a handmaiden for protection. The Droid Army captures the decoy Queen, so Padme was in the right to be concerned for her safety.
The chaos surrounding the Trade Federation and Separatist Movement puts a large strain on the safety of each senator.
Padme fears for her safety, sets up a decoy to cover for her
In Star Wars: Episode I – The Phantom Menace, Padme chooses to have a decoy take her place. She suspects the oncoming chaos from the Trade Federation will affect her safety. Earlier in the film, the Queen’s decoy is captured by the Droid Army and pressured to sign a treaty that will legitimize the invader’s occupation.
The Trade Viceroy, who is working with the Trade Federation, has assurances that the Droid occupation will be ratified by the Senate once approved by Amidala. The decoy Queen refuses, but the Viceroy insists that the suffering of her people will persuade her to see the Federation’s point of view.
This is the same Viceroy who attempts to end Qui-Gon and Obi-Wan with poisonous gas. The two Jedi attempt to negotiate the trade dispute that kicks off the conflict, but are attacked for their efforts instead.
Padme reveals she is pretending to be the handmaiden
Once Qui-Gon and Obi-Wan reach Theed, the capital of Naboo, they are successfully able to rescue the decoy Queen and escort her to Coruscant.
Decoy Queen Amidala, representing the Galactic Republic, asks the Gungans for support. She wishes to form an alliance with Boss Nass, leader of the Gungans.
As the Queen begins her proposition, she is quickly interrupted by the handmaiden. The maiden reveals herself to be Padme, the true Queen of Naboo. The woman impersonating her is a decoy.
She apologizes for her deception, but insists it was necessary to protect herself. Although the Republic and Gungans do not always agree, Padme says, the two great species have always lived in peace.
Padme continues, saying the Trade Federation has destroyed all that the Republic and Gungans have worked so hard to build. If they do not act quickly, all will be lost forever. She then kneels and begs for the boss’ help.
The rest of the group follows, kneeling as well. Padme says their fate is in the Gungans’ hands.
So that’s the answer. Padme chooses to pretend to be a handmaiden for her own safety. The Trade Federation and Droid Army attempt to kidnap the Queen, so she’s in the right to temporarily hide her true identity.