Superconducting cavities are widely used in linear accelerator facilities all over the world. It is challenging for the low level RF (LLRF) system to stabilize the RF field inside the cavities working in pulsed mode with heavy beam loading and strong Lorenz force detuning. Feedback loops are implemented in LLRF system which should have fast responses to the disturbances on cavities. To enhance the feedback performance, active compensation of Lorenz force detuning with Piezo tuners and the compensation of beam loading with feed forward control are strongly needed. They all require the knowledge of cavity parameters, such as the quality factor, time varying detuning and the amplitude and phase of the beam. A disturbance observer based on the active disturbance rejection control (ADRC) was designed for superconducting cavities counting the Lorenz force detuning and beam loading as disturbances to the cavities. The ADRC disturbance observer can be integrated into LLRF feedback loops to speed up the responses to disturbances and the outputs of the observer are interpreted furtherly to identify the cavity parameters. This paper presents the algorithm and architecture of the ADRC disturbance observer, the simulation of the LLRF feedback loop with disturbance rejection and the cavity parameter identification applied on the archived data of superconducting cavities in operation.