The invention discloses a preparation method of a diamond coated high-temperature high-pressure spray nozzle. The preparation method comprises the following steps: taking a pretreated hard alloy or SiC/Si3N4 ceramic nozzle as the coating substrate material, depositing a diamond coating on the surface of the inner hole of the nozzle by a double-substrate straight-pull perforating heating wire arrangement method, and depositing a diamond coating on the conical surface of the inlet of the nozzle by a parallel heating wire arrangement method. The deposition of the diamond coating even in thickness and mass and excellent in erosive wear resistance on the whole working surface of the high-temperature high-pressure spray-drying nozzle is realized by a heating-wire chemical vapor deposition method and through the technical scheme of two-step wire arrangement deposition, and therefore, the service life of the previous nozzle is greatly prolonged; besides, the preparation method is significant for ensuring safe operation of the spray equipment and improving the product quality.