The invention discloses a slag thickness detection and slag adding prediction method and system. The method comprises the following steps: obtaining the real-time measurement data and the corresponding real-time auxiliary data of each slag point on the surface of a crystallizer protection slag layer; utilizing the real-time measurement data and the auxiliary data of each slag point to calculate a corresponding real-time slag thickness value of each slag point on the surface of the slag layer; on the basis of the change situation of the real-time slag thickness value of each slag point on the surface of the protection slag layer, predicting the position on which slag needs to be added, and slag adding time by taking a preset slag thickness value as a reference. Therefore, according to the invention, the slag thickness value of the protection slag layer does not need to be subjected to manual measurement, defects of big manual workload, big measurement error, no possibility of on-line measurement and the like in the prior art can be overcome, and an efficient and reliable guide function is provided for slag adding operation.