A farm sensing system is provided. The farm sensing system includes a cloud server, a sensing apparatus, and a computer device. The sensing apparatus is configured to be connected to a specific sensor disposed on a farm. The computer device is configured to obtain specific sensor data generated by the specific sensor through the cloud server. In response to there being potential failure of the specific sensor, the sensing apparatus enters a sensor-calibration mode. In response to a reference sensor being connected to the sensing apparatus, the sensing apparatus builds a calibration table by periodically receiving specific sensor data and reference sensor data, and executes a finite-state machine to perform a calibration procedure on each entry in the calibration table. In response to the number of consecutive hits of the one-to-one correspondence in a specific entry in the calibration table reaching a predetermined number N, the finite-state machine stops the calibration procedure and determines that the specific entry is calibratable.