In this paper, an ultrasonic fixed-point liquid level monitoring sensor for airborne wastewater storage tanks is designed. The sensor adopts a fully enclosed structure, which effectively eliminates the adverse effect of the environment on the monitoring process. At the same time, a dual decision algorithm is proposed to improve the accuracy and reliability of the monitoring. The effectiveness of the sensor is verified. The theoretical and practical analysis shows that the sensor can monitor and alarm the fixed-point liquid level more accurately and stably.