Abstract: Through researching on a boiler furnace slagging, the on\|line furnace heating surface slagging monitoring system based on acoustic pyrometry and artificial neural network is put forward. The method combines the non\|contact temperature measuring tool, acoustic pyrometry device, with BP neural network. The acoustic pyrometry device can measure the furnace outlet gas temperature, which provides the precondition for the furnace slagging on\|line monitoring.It can make use of BP neural networks nonlinear mapping calculation of the furnace ideal outlet gas temperature, and estimate slagging of the whole furnace with the pollution coefficient. To verify the method of slagging on\|line calculation, the data of furnace ideal outlet gas temperatures from a power plant between on\|line acoustic pyrometry and BP neural network prediction can meet the project requirement and the relative error is about 3%.