In this paper, we study the thermomagnetic power generation performances of compound series Mn1.2Fe0.8P1-xSix in a strong magnetic field of permanent magnet. The compounds are synthesized by using the high-energy ball milling and solid state reaction method. The crystalline structures, magnetic properties, and the thermomagnetic power generation performances of the compound series Mn1.2Fe0.8P1-xSix are measured. The results show that Mn1.2Fe0.8P0.37Si0.63 and Mn1.2Fe0.8P0.35Si0.65 are the compounds of a first-order phase transition and the soft ferromagnetic materials, and they are of Fe2 P-type hexagonal structure, Curie temperatures of these compounds are 334 K and 348 K in the industrial waste heat temperature zone. According to this feature that temperature variation of the first-order phase transition material leads to a large change of magnetization at the Curie temperature, we design a demonstration device for thermomagnetic generator, and measure the relationships of induction current generated in ferromagnet phase transition with the coil turn number, mass and surface area of thermomagnetic generator material, and the gradient of surface temperature for compounds Mn1.2Fe0.8P0.37Si0.63 and Mn1.2Fe0.8P0.35Si0.65. The results show that the Mn1.2Fe0.8P1-xSix compound series possess the high performances of thermomagnetic power generation, and they are expected to be candidates of magnetic materials for thermomagnetic power generation.