Abstract: This paper presented a fixed-point liquid level measuring method using ultrasonic pulse reflection to detect the dose of warship halon fire-extinguishing systems. When ultrasonic wave travels through the gaseous medium and liquid medium，the attenuation rate of the remnant ultrasonic signal was found to be distinct，which was reflected by the different envelope area of the return remnant signal. In this way， the liquid level could be easily defined. Firstly，the received return signal was de-noised using the wavelet analysis theory，and the resulted low-frequency signal was reconstructed into a more realistic signal. Next， a threshold was proposed to judge whether the medium at the detecting point is gaseous or liquid. Finally， the envelope area of the remnant wave signal was examined and compared with the threshold，through which the liquid level can be precisely determined. Generally，all liquid level measuring problems in gas containers with pressure systems similar to fire extinguishing agents 1301，which are widely adopted as marine fire extinguishers，could be easily solved by this method.