She sought bail for her son on Friday, the 10th such request, and shaved her head in front of the court to protest what she said was injustice. On Thursday the court rejected a bail request, saying its previous reasons still applied.
Each perceived insult of Thailand's monarchy can be punishable by up to 15 years in prison under its lese majeste law.
"I am just a mother who loves her child. My son did not do anything wrong, he only has a different opinion. He has not received justice," she said.
The court set a bail hearing for May 6, which his lawyer Krisadang Nutcharat said was a positive sign because the court previously did not set hearing dates.
Parit emerged as a leader in last year's youth-led protests against the government of retired general Prayuth Chan-ocha, who seized power from an elected government in a 2014 coup and stayed on as prime minister after a 2019 election.
Protest leaders have also called for reform of the monarchy and abolition of the lese majeste law. Legal action is being pursued against dozens of demonstrators over those calls.