<p>Phosphate, as an inorganic binder, is highly environmentally friendly and holds great promise for future advancements in foundry binders. However, its broader application is constrained by current limitations in phosphate self-hardening sand methods, such as slow curing times and insufficient mold strength. As a key component of phosphate self-hardening sand technology, optimizing the formulation of the curing agent can significantly enhance the performance of the self-hardening sand process. To address the challenges of slow curing rates and inadequate mold strength in phosphate-based self-hardening silica sand, this study focused on optimizing an existing single-component fused magnesia curing agent. This was accomplished by incorporating finer metallurgical magnesia powder and microsilica powder into a composite formulation, leading to the development of a novel powdered curing agent. Experimental results demonstrate that formulating the composite powder curing agent with fused magnesia powder (200 mesh), microsilica powder, and metallurgical magnesia powder (1000 mesh) in a ratio of 8:1:1.5, and incorporating it at a dosage of 1% by sand weight, yields tensile strengths of phosphate self-hardening sand of 0.30, 0.76, and 1.23 MPa at 1, 4, and 24 h, respectively. The rate of hardening of phosphate binders and the strength of sand molds have been markedly enhanced. The accelerated curing mechanism of the composite hardener was investigated using analytical techniques, including Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The results demonstrate that the composite curing agent—comprising fused magnesia powder, microsilica powder, and metallurgical magnesia powder—significantly enhances the tensile strength of specimens and accelerates the hardening process of phosphate self-hardening sand. This study provides a theoretical foundation for the rapid and high-quality development of phosphate self-hardening sand systems in future foundry applications.</p>