A flat luminescent lamp and a method for manufacturing the same are disclosed in the present invention. The product produces foam in an outstanding manner without the need for a propellant gas and is satisfactorily worked into the hair without troublesome after-foaming. 70 wt %, which is higher than that of the naphthalene-based water reducer and the sulfonated lignin water reducer prepared by oxidation and sulfonation of alkali lignin, and 28-days compressive strength ratio of the concrete with this water reducer to the concrete without this water reducer is up to 150:100.