Trey Songz got off pretty light after going crazy and tearing up a stage at his show in Detroit ... injuring a cop in the process.

The singer pled guilty to 2 misdemeanor charges of disturbing the peace for destroying gear and throwing debris when his set was cut off at Joe Louis Arena in December. Charges of aggravated assault and resisting and obstructing a police officer were dismissed as part of his plea ... according to Wayne County prosecutors.

Trey was sentenced to 18 months probation, and has to go to anger management classes and submit to drug tests. He also has to pay restitution.

Gotta wonder, though ... does he still think it was worth it?

FIT OF RAGE