The gravitational wave detection program provides a new and effective way of observing the universe, which helps to reveal the basic operating laws of the universe. The production process of a Au-Pt alloy, a key component (“test mass”)for space-based gravitational wave detection, was explored, thus compensating for the lack of research results related to the multiscale solidification process of Au-Pt alloy castings. Macroscopic finite element calculations reveal that the alloy melt experiences strong convection at the beginning of filling and then gradually tends to stabilize, resulting in annular flow in the mold. During the solidification process, the temperature gradient around and at the bottom of the casting is large, whereas the temperature gradient in the center of the casting is small, resulting in an obvious temperature stratification phenomenon. The solidification starts from the bottom and the surrounding area to the center area. In the early stage of solidification, a layer of fine crystals appears on the surface of the casting. During the process of the solidification of the casting advancing inward, the columnar crystal area is formed, and finally, equiaxial crystals are formed in the center of the region. The influence of different casting processes on the quality of castings was explored. With increasing pouring temperature, the air gap and pressure gradually decrease, and the shrinkage phenomenon is improved. However, when the temperature is increased to a certain degree, the improvement effect becomes less obvious. The change in pouring speed directly affects the kinetic energy and momentum of the alloy melt. The flow is not obvious when a low speed produces a temperature stratification phenomenon and a grain coarsening phenomenon, whereas an appropriate increase in speed makes the organization more uniform and denser. In addition, the heat transfer coefficient of the mold affects the solidification time, microstructure morphology and number of grains in the sample.