This work aims to address the severe film detachments from the surface of boiler steel in thermal power plants after steam corrosion attacks, especially as the accumulated detached oxide layers are prone to block boiler tubes, reducing their heat transfer efficiencies. The operational conditions of a typical thermal boiler were simulated with TP347H steel as the experimental material. Gravimetry, metallographic analysis, and scanning electron microscope were employed in the corrosion mechanism investigation of the boiler steel in high temperature steam, and the growth and shedding mechanism investigations of the corrosion product layers. The TP347H steel was rapidly oxidized in dry steam at 650 °C to generate Mn and Cr oxides, whereas Fe²⁺ was formed after the corrosion process. FeO and FeOOH layers were crystallized out of the Mn and Cr oxides, and the bond strength between the Mn and Cr oxides and the FeO and Fe₂O₃ layers was weak, making the detachments of the FeO and FeOOH layers quite easy. Continuous oxidation was observed when the oxidized film layer was exposed to the atmosphere, with some parts of the FeO in the film layer readily oxidizing to Fe₂O₃. This increased the stress in the film, thereby promoting the film detachment during exposure to the atmosphere. This research provides a theoretical basis for the corrosion protection of boiler tubes and their safe operations.