Video shows campus police spraying peaceful Occupy protesters last week.
UC President Mark Yudof asked Bratton to head up an investigation and gave him 30 days to complete his report.
State lawmakers are looking into the matter as well.
UC Davis Chancellor Linda Katehi, who has faced criticism from students, faculty and staff in recent days, said she had instructed campus police not to use force in breaking up the protest.
An officer and the campus police chief have been placed on paid administrative leave in the wake of the incident at the 32,000-student campus that lies west of the state capital.
The Associated Press contributed to this report.