In order to achieve online monitoring about hot-state dimension of large-scale forgings, a new monitoring method has been developed in this paper. The large-scale barrel-type forgings heat conduction differential equation is deduced on the basis of the heat transfer. The functional form of heat conduction equation for large-scale forgings is established. Then the relationship between temperature and dimension is solved according to the variational principle. So the dimension of the large-scale forgings is monitored by combining the relationship between temperature and dimension. The temperature is measured by sensor. As a result, the online monitoring about hot-state dimension of large-scale forgings is achieved.