A safe and effective method for the horizontal synthesis of polycrystalline indium phosphide is provided in this work. The thermodynamic analysis and experimental verification in the temperature range of 923 K–1073 K are used to evaluate viability and the reaction route of the method. The polycrystalline InP were characterized by the X-ray diffraction (XRD) in combination with the Rietveld method, scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) and inductively coupled plasma-optical emission spectrometry (ICP-OES). The results show that the purity was improved and the efficiency synthesis was achieved 99.55% under the optimum condition of ω (In/P) = 1:4, P = 10 Pa, T = 998 K and t = 30 min by this method. The synthesis mechanism was also elucidated in this work, and the synthesis process was primarily controlled by the diffusion of P2 vapor.