The Healthcare Commission is looking into 'serious concerns' from both within and outside the trust over clinical governance, management and safety arrangements.
It will establish whether the trust has adequate patient safety systems and processes in place.
Healthcare Commission head of investigations Nigel Ellis said: 'Concerns about the trust have been brought to us from a number of different sources.'
NHS West Midlands chief executive Cynthia Bower said: 'We are committed to making sure improvements in care continue and that these are underpinned by legal safeguards to protect patients and staff.'
Last May, the trust's maverick chief executive Roger Thayne stood down, claiming the service was 'in crisis' after the government announced long-term plans to merge Staffordshire's ambulance services with those of the rest of the West Midlands.
In August, the Department of Health said the high-performing trust was not recording response times in line with official guidance.