It follows an unannounced inspection at the hospital in Dundonald in February.
In every area inspected, there were concerns about nurse staffing levels, patient care records and training for staff, said the RQIA team.
The report is part of the RQIA's programme of unannounced inspections across all hospital sites.
Its inspection team were advised that there was an insufficient number of junior medical staff at the Ulster Hospital, particularly at nights and weekends.
The report, however, said staff were compassionate and showed empathy to patients at all times.
The RQIA's Olive MacLeod said staff in the emergency department raised concerns about:
"RQIA's inspectors also highlighted essential equipment was not always available at busy times  and designated rooms for vulnerable patients could not always be guaranteed," she said.
"We were also concerned that routine safety rounds for patients were not carried out."
Mrs MacLeod said the RQIA recognised that increases in the number of admissions placed additional pressure on staff in ensuring the provision of safe, effective and compassionate care.
"However, it is vitally important that this continues to be delivered - even at times of such pressures," she added.
In response to the report, the South Eastern Health Trust said: "Many of the concerns raised by the inspection team were issues that we were already aware of and were working to address.
"This work continues and since the inspection, has included an increase in senior nurse leadership in our emergency department and an overall improvement in permanent nurse staffing.
"There are, however,  a number of factors which are relevant to this report, both regionally and within the trust.
"This inspection was carried out on one of the busiest weeks of the year, when all hospitals were under extreme pressure.  There is a well recognised capacity issue within the Ulster Hospital."
Concerns about staffing levels at the Ulster Hospital have been raised by the health regulator, the Regulation and Quality Improvement Authority (RQIA).