Unison said a commitment to maintain officer numbers had been accompanied by huge cuts in the number of civilian jobs.
It also said it was “disappointed” with the Scottish Police Federation (SPF) for failing to represent the whole of the “police family”.
Earlier this week, the SPF said Police Scotland was at “breaking point” after officer numbers fell to their lowest level since 2010.
Unison Police Staff Scotland (UPSS) said the views of the SPF “failed to encapsulate” all the problems being faced by the national force.
The union said police officers were being moved into backroom roles to compensate for the loss of 2,000 police staff since the creation of Police Scotland in 2013.
In a statement, it said: “The issues affecting organisational performance and operational deployment are not purely as a result of officer numbers falling to their lowest levels for five years and cannot be viewed in isolation.
“The problems stem from how the organisation has been restructured since the creation of Police Scotland - with the primary intention being the release of as many police staff through voluntary redundancy and early retirement as could be justified to meet target savings.”
It added: “As a consequence, our members and those of the SPF are operating under tremendous strain due to the massive increased burden of work without a wider recognition of the long-term implications.
“The ongoing failure of the organisation to fill police staff vacancies and the continuing deletion of posts from already restructured departments is driving standards down whilst turning the pressure up on staff and Unison views ongoing developments as completely unsustainable.”