An officer has opened fire as a teenager allegedly tried to run down his partner during a traffic stop in Western Australia's south yesterday.

About 6:30pm on Tuesday, police pulled over a vehicle on the Coalfields Highway to the west of the town of Collie, about 200 kilometres south of Perth.

As an officer got out to speak to the driver, the man allegedly reversed and tried to run the policeman down.

The police officer jumped onto the bonnet of the car, but was thrown off as it reversed again, crashing into the police car parked behind it.

As the car started to drive towards the fallen officer again, his partner took a desperate shot at the car's tyre.

The tyre was damaged but the car still drivable.

Police pursued the vehicle, which stopped a short distance away and the driver was arrested.

The police officer, a first class constable, was treated at Collie Hospital for injuries to his back.

Police have charged a 17-year-old Collie boy with acts likely to endanger the life, health or safety of any person and driving-related offences.

WA Police would not say why officers pulled the car over.

The road was closed for some time, but has since been reopened.