The management committee of the Taishan Scenery Area said the lion broke loose in Tai'an Tiger Mountain Park, a scenic spot, when the trainer entered its cage for cleaning.
The trainer died in the hospital.
The zoo called police to shoot the lion in order to ensure the safety of tourists.
Police are still probing the incident to get more details, state-run Xinhua news agency reported.
A similar animal attack happened in another Chinese zoo in March this year, when a zookeeper was mauled by a tiger at a zoo in east China's Jiangxi province.