The boy, who is unable to communicate, has been taken into protective custody with his younger siblings following their parents' arrest, said police spokesman Bob Dunn in Anaheim, southeast of Los Angeles.
The cage, which had a mattress inside, was of the type normally used to house a pet, he said, adding that there were "varying reports" of how long the boy had been caged, "from several years to several months to several hours."
"Perhaps because of some of the outbursts that occurred because of his autism... the family was having difficulty coping with that and controlling that," Dunn told KTLA television.
"Putting him in that cage may have been a way for them to try to, in their way, control what was going on," he said after the arrests on Tuesday night.
The Vietnamese parents -- who speak English, but not fluently -- were detained on suspicion of child endangerment and false imprisonment, said the spokesman. The boy and his siblings -- a boy and a girl, aged 10 and eight -- were in good physical condition.
"Other than perhaps living in that cage, there was no other signs of any type of abuse that were visible to the officers," said Dunn.