A large-scale multi-step synthesis method for ultralong silver nanowire with controllable diameter, comprises: an ethylene glycol solution containing polyvinylpyrrolidone and sodium chloride is fully heated to obtain a solution with strong reducibility, and then silver nitrate in glycol solution is added for a generation of a large number of crystal seeds; hydrogen peroxide is used to achieve the selection of the crystal seeds for a small amount of crystal seeds with particular sizes; the temperature is immediately raised to increase the reaction rate until the threshold of the etching crystal seeds of nitric acid is broke through; the temperature is lowered for long-timed reaction to slow down the reaction rate, reduce the probability of the isotropic seeds by self-nucleation and promote the absorption of small nucleus in the radial direction of large nucleus or nanowire, thus obtaining the ultralong silver nanowire.