The invention discloses a locking device capable of maintaining the three positions of a handle for a low-voltage circuit breaker and a method thereof. The locking device comprises a support and a retainer, wherein the retainer is formed by connecting two supporting parts and a locking part; a convex surface and a concave surface corresponding to the convex surface are arranged right in the middle of the locking part; a positioning groove and a positioning platform in clearance fit are arranged on the supporting parts; the support is fixed on the top cover of the circuit breaker, and the handle of the circuit breaker in the switching-on position extends upwards out of the support, so the handle of the circuit breaker can be locked in different positions by changing the placement of the retainer, wherein the placement direction of the retainer in the switching-off position is the same as that of the retainer in the trip-free position, while the acting surface of the retainer in the switching-off position is different from that of the retainer in the trip-free position, and the placement direction of the retainer in the switching-on position is opposite to the placement directions of the retainer in the switching-off and trip-free positions; and two padlocks are respectively inserted into two padlock holes of the support and further locked. Accordingly, the handle of the circuitbreaker has universality and reliable locking-up effect in the switching-on, switching-off and trip-free positions, thus achieving the multi-user and multi-lock control.