The Norfolk and Suffolk Foundation Trust wants to cut its deficit from £8.9m in 2015-16 to £6.1m in 2016-17.
The trust says in its report it hopes most posts can be cut through "non-essential" vacant posts being left unfilled.
But Unison claimed it would have a "dramatic impact on services".
The trust's annual operational plan for 2016-17 says demand for mental health services "continues to increase, whilst our funding declines".
The trust, which has an annual budget of more than £200m, added: "While we must control our own costs, we recognise that 'parity of esteem' is yet to be reflected in parity of resources.
"Our Board will continue to campaign to improve this position.
"We have a robust financial recovery plan which will see us deliver a £6.1m deficit in 2016-17. Like the wider health and social care community, our trust faces long-term challenges, often outside of our control."
The trust had planned to cut 135 posts, but said that it had now reduced that figure fo 90.
Anne Humphrys, from the Campaign to Save Mental Health Services in Norfolk and Suffolk, said: "The trust is in special measures because it was rated inadequate.
"Further cuts is no way going to improve the service. There are not enough staff to meet the needs of the service at present."
Jeff Keighley, of the Unison union, said staff were "trying to run a service without the adequate resources and for them it's quite frightening".
He added: "It will have a dramatic impact on services."
The foundation trust said it did not plan to make any compulsory redundancies.
It was placed in special measures by the Care Quality Commission in 2015 when it was described as "not a safe... service".
An NHS trust caring for mentally ill people in Norfolk and Suffolk which was placed in special measures by a care watchdog, is planning to cut 90 posts.