The Central Commission for Discipline Inspection announced the probe in a statement, without giving details about the investigation.
Sun was removed from his position this month as head of monetary policy department at the People's Bank of China (PBOC), which he had held since 2018, the anti-graft body said.
The Economic Observer newspaper reported on Wednesday that Zou Lan, head of financial markets at the PBOC, would be named the new head of the monetary policy department, replacing Sun.
Sun was head of the PBOC's financial research institute from October 2016 to August 2018, it said.
Sun's removal is expected to have little impact on monetary policy. Compared with Western central banks, the PBOC still has limited policy independence despite growing influence. The basic course of monetary and currency policy is set by the cabinet or by the ruling Communist Party's Politburo.
Chinese President Xi Jinping has stepped up efforts in recent years to weed out corrupt Communist Party officials in the financial sector.
In February, the anti-graft watchdog concluded an inspection of institutions including the central bank, the banking and insurance regulator, stock exchanges, commercial banks and asset-management companies, in the country's first coordinated inspection of the sector since 2015.
(Reporting by Kevin Yao and Beijing newsroom; Editing by Jason Neely, Kim Coghill, William Maclean)
