Legal Question in Criminal Law in California

That sounds like a switch blade knife and those are illegal.

Your question doesn't give too much information but Penal Code Section 17235 governs switch blades. I've provided a copy. If the knife is covered by the exception discussed in the second part of the section. It may be legal.

." As used in this part, "switchblade knife" means a knife

having the appearance of a pocketknife and includes a spring-blade

knife, snap-blade knife, gravity knife, or any other similar type

knife, the blade or blades of which are two or more inches in length

and which can be released automatically by a flick of a button,

pressure on the handle, flip of the wrist or other mechanical device,

or is released by the weight of the blade or by any type of

mechanism whatsoever. "Switchblade knife" does not include a knife

that opens with one hand utilizing thumb pressure applied solely to

the blade of the knife or a thumb stud attached to the blade,

provided that the knife has a detent or other mechanism that provides

resistance that must be overcome in opening the blade, or that

biases the blade back toward its closed position."