It happened around 9:30 p.m. at E Street north of Olive in the city of Madera as the officer responded to a call.
The officer totalled the patrol car when it crashed into the underside of the trailer.
He was taken to Community Regional Medical Center in Fresno with serious injuries but was released Sunday.
The officer is expected to be okay.
Police say he did not have his lights or sirens on as he responded to the call.