Dyfed-Powys Police requires improvement in three key areas, an annual assessment has concluded.
HM Inspector of Constabulary Wendy Williams said she was "concerned about many aspects" of the force's performance in 2016.
In the annual PEEL report, Ms Williams said she had been in "regular contact" with the chief constable in view of her findings
Dyfed-Powys Police said it had already begun to make changes.
PEEL assessments look at the effectiveness, efficiency and legitimacy of all the forces in England and Wales.
It is the second year running Dyfed-Powys Police has been assessed as needing to improve.
Ms Williams said she was "pleased" it was addressing issues raised in previous inspections but she "did not underestimate" how much the force needed to improve to provide a consistently good service.
She said the way officers were dispatched to investigate whenever a crime is recorded meant that the force may not be making best use of officers' time.
"Cases are allocated for investigation to whichever officer happens to be available rather than to investigators with the appropriate skills and experience," she added.
"This means that some investigators may not be properly equipped to deal with all the cases they are allocated."
The force had a good approach to preventing crime, was good at tackling serious and organised crime, and had a good track record of achieving financial savings, the report said.
But Ms Williams said she was "concerned that it still does not have a comprehensive understanding of the costs, or quality, of the service provided through its current operating model".
"The force does not have a sound understanding of the skills and capabilities of its workforce," she added.
She also raised concerns about gaps in the force's understanding of the extent to which corruption is having an impact on the organisation, and of the extent to which officers may be abusing their authority for sexual gain.
Darren Davies, deputy chief constable of Dyfed-Powys Police, said it was important to note the inspection was conducted last year.
"We've got a new chief officer team in place now and a new police and crime commissioner who has set out clearly the direction for the future," he said.
"We welcome the report but also we've made strides and are continuing to improve on things.
"We've implemented a number of changes which address the recommendations in the report… so I'm pretty confident that when this year's round of inspections occur we'll see an improved picture."