316H austenitic stainless steel used in nuclear power has higher requirements for material magnetism, and the main magnetic phase is ferrite. Thus, the ferrite content in 316H steel castings needs to be strictly controlled. This study investigated the ferrite phases in 316H austenitic stainless steel electro-slag remelting (ESR) ingot used in nuclear power applications. The morphology, content and decomposition of ferrite as well as microsegregation in 316H ESR ingot are studied by optical microscopy (OM), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and electron probe microanalysis (EPMA). The solidification process is calculated by thermodynamic calculation using Thermo-Calc. Besides, the empirical chromium and nickel equivalent formulas and phase stability diagram are used for prediction of the solidification mode and ferrite content. The experiment results show that the ferrite morphology changes greatly in the direction of the thickness of ESR ingot. From surface to center of the ESR ingot, the ferrite morphology varies as granular → short rod-shaped → blocky → skeletal → parallel short rod-like + network-like. The ferrite content in the thickness direction of the electroslag ingot varied from 1.92 to 3.57 %, showing an “A” shape distribution. There are three kinds of decomposition behavior of ferrite: decomposition into Sigma phase and austenite phase by the eutectoid reaction, transformation into Chi phase in regions enriched with high Mo element along grain boundaries, and direct transformation into secondary austenite phase. In addition, the thermodynamic calculation and most of empirical formulas predicts the solidification mode is FA mode. From the microstructure analysis, the solidification mode of ESR ingots change from AF mode to FA mode from surface to center. Furthermore, the formulas proposed by Hull provides most accurate predictions results in ferrite content.