The invention discloses a device capable of monitoring the rotation angle of a swing mechanism in real time and a monitoring method thereof, and belongs to the field of engineering machinery. The device comprises a concrete pump truck, a rotary supporting mechanism, a speed reducer, a rotating speed sensor and a monitoring terminal. The rotary supporting mechanism is installed on the concrete pumptruck, the speed reducer is installed on the rotary supporting mechanism, the rotating speed sensor is installed on the speed reducer, a first gear is arranged on the rotary supporting mechanism, a second gear is arranged on the speed reducer, the second gear is meshed with the first gear, and the rotating speed sensor is electrically connected with the monitoring terminal. The device is simple in structure. The rotating speed sensor is directly installed on the speed reducer, and the rotating speed sensor and the speed reducer are organically combined, so that the rotating speed sensor can output stable signals and is not easily interfered by the outside. Parameters of the speed reducer and output signals have a stable conversion relation, so that results obtained through real-time monitoring are accurate and can be visually observed through a computer, and data collection is quite convenient.