The invention relates to the field of low-voltage electric appliances, and more particularly, to a miniature circuit breaker, which includes a button mechanism, a circuit breaker housing, and an anti-switching-on mechanism, wherein the button mechanism is arrange at one end of the circuit breaker housing and is in sliding fit with the circuit breaker housing, the anti-switching-on mechanism includes a first locking member, the first locking member is pivotally arranged on the circuit breaker housing, one end of the first locking member is in locking fit with the button mechanism to prevent the button mechanism from moving towards a switching-on direction, and after the miniature circuit breaker is assembled to an assembly position of the circuit breaker and installed in place, a housing at the assembly position of the circuit breaker acts on the first locking member, so that after the first locking member is unlocked from the button mechanism, the button mechanism is capable of moving towards the switching-on direction for a switching-on operation; and according to the miniature circuit breaker of the invention, the anti-switching-on mechanism thereof ensures that the miniature circuit breaker is capable of being switched on only after being assembled to the assembly position of the circuit breaker and assembled in place, thus ensuring the use safety.