They say she waved a replica gun at them when they tried to talk to her.
Investigators say an officer fired one round, hitting her in the chest.
Sheriff's detectives say the replica looks just like a real gun, right down to the moving parts.
She was rushed to a local hospital where she was pronounced dead, authorities said.
Officers said there was no way of knowing whether the gun real or not.
"In this case, this replica is virtually indistinguishable from a regular firearm. It shares all the same characteristics of a regular-looking deadly gun," said Lt. Dave Dolson from the Los Angeles County Sheriff's Department.
The shooting is the latest in a string of recent officer-involved-shootings involving the CHP. In fact, this is the third CHP shooting in the past two months.
In May, there were two fatal shootings. One on 7th and Vermont, when authorities said the suspect pulled out a gun on officers. Then in Covina, officers fired when a suspect rammed a stolen car into patrol cars.