In iron and steel industry, production process is accompanied by a large amount of residual heat as high temperature slag. Methane reforming with carbon dioxide is one of the typical chemical energy storage processes, and it can be applied to use residual heat and reduce carbon dioxide emission. In this paper, thermochemical energy storage performance of methane reforming using high temperature slag is researched. According to experimental and numerical results, high temperature slag can be used as energy source and catalyst for thermochemical energy storage process by methane reforming. Slag is almost non-porous material, and its activation energy is higher than that of common catalyst, so slag only has high catalytic activity under high temperature. During methane reforming process, methane conversion and thermochemical storage efficiency first increases and then decreases with reaction rate dropping, and the position with maximum reaction rate gradually changes from front of slag bed to the end. Many factors including inlet conditions and reactor structure can affect thermochemical storage performance. Increase of slag initial temperature can improve methane conversion and thermochemical energy storage efficiency. As reactant flow rate decreases or slag bed length rises, methane conversion gradually increases, while thermochemical energy storage efficiency first increases and then decreases. With suitable conditions, thermochemical energy storage efficiency of slag can be higher than 60%.