Method for changing solid deslagging into liquid deslagging of once-through boiler

A method for changing solid slag discharge into liquid slag discharge of a once-through boiler comprises the specific steps that a combustor, a lower water-cooled wall, a dry bottom hopper, a water-cooled wall lower collecting box and a slag conveyor of an original once-through boiler are omitted, a double-liquid slag discharge boiler is additionally arranged on the lower portion of the original once-through boiler, and a saddle arch is arranged at the bottom of a lower cooling chamber; a hearth of an original once-through boiler is transformed into an upper cooling chamber of a liquid deslagging boiler, the front wall and the rear wall or two side walls of a water cooling wall shrink inwards and are in butt joint with a lower cooling chamber, and a slag catching pipe bundle transversely penetrates through a combustion chamber outlet flue to catch liquid slag carried in smoke; pulverized coal is combusted in the combustion chamber through the combustor, formed ash slag is molten liquid slag, and the liquid sl