The invention relates to a preparing method of an astronavigation tablet vanadium pentoxide. The preparing method is characterized by comprising the following steps of adding water to leach vanadium slag after the vanadium slag is roasted by sodium oxide, adjusting the pH till the solution is alkaline, adjusting the pH again till the solution is faintly acid after iron is removed, adopting an aluminum method for impurity removal, after filtering, adding a polymeric fluorescent to remove impurities, afterwards, adjusting the pH till the solution is strongly acid, heating and adding precipitator, washing, filtering, drying and roasting to obtain powdery vanadium pentoxide, and preparing the product meeting the standard of the stronavigation tablet vanadium pentoxide after melting and coolingtableting.