Up to the end of 1994, the total installed capacity of generating units of the East China Power Network now aggregates to 36000MW among which more than 20000MW is of units with capacity above 100MW and with high steam parameters. In recent years, 6×600MW units with supercritical and subcritical steam conditions were put into operation in the Network. In connection with operation situation of 6×600MW boilers, the most important problem is furnace slagging, which seriously jeopardizes on safe and economic operation of the boiler unit. Investigation for furnace slagging situation of 6×600MW units, reasons of slagging and precaution measures of slagging are described in this paper. By these means mentioned above, the load of the boiler will be maintained and forced outage will be avoided.