A floating-type auto-cleaning robot was designed to solve the cleaning conundrum of the inner wall of large liquefied natural gas (LNG) tanks. Such a cleaning robot was designed based on the modularization, wherein all modules could enter the LNG tank through the slinging hatch at its top and were assembled on the tank’s inner wall before performing cleaning work. On the base of an introduction to the makeup and operating principle of the cleaning robot of the inner wall of LNG tanks, this paper analyzes its navigation control, autonomous navigation, stability design, system integration, and other key technologies. The robot’s control performance was tested by a navigational stability experiment; the cleaning-related technological parameters of the cleaning robot were verified by a cleaning effectiveness experiment. Both experiments demonstrated the cleaning robot could perform safe, reliable, and high-efficient cleaning of the inner wall of LNG tanks in a harsh environment.