A type of liquid level gauge for continuous measurement of boiler steam drum under full conditions and its method for calculation of liquid level, this liquid level gauge includes one metal measurement tube and one transmitter. The steam-side pipe connected to steam drum is fitted on the upper part of the metal measurement tube and the water-side pipe connected to steam drum is fitted on the lower part of the metal measurement tube, one insulated jacket pipe is available inside the metal measurement tube and 1# probe, 2# probe, and 3# probe are mounted from down upwards inside the insulated jacket pope. The respective probes are mutually insulated and not conducted. The three probes are respectively connected with the transmitter through lead wires of the probes. This liquid level gauge is able to monitor the water and steam dielectric constants in a real-time manner, and the liquid level values can be outputted accurately in a continuous way with the liquid level gauge for boiler steam drum designed according to this invention in all the cases, no matter the boiler is under the startup, shutdown, or joint discharge status, etc, thus to achieve the function for a continuous measurement of liquid level under full conditions.