The present invention relates to a feedstock for 3D printing, a preparation method and an application thereof. The feedstock is polymer binder-coated metal powder, being in a linear shape. After being printed into a green body with a preset shape via a 3D printer, the linear feedstock is sequentially degreased and sintered, so that a metal product with a complex structure and high accuracy can be obtained. Compared with the prior art, the linear feedstock is applied to 3D printing, so that waste of raw materials can be avoided; the accuracy of a product surface is controlled and the quality of products is improved by selecting different wire diameters of the feedstock and controlling the heating temperature; and melting treatment can be performed by a simple thermocouple without need for complex and dear laser heating equipment, so that production cost is reduced. The powder injection molding technology and 3D printing technology are combined, so that complex products can be quickly printed and manufactured, development flow is shortened, and mass production popularization is realized. The feedstock has good economic benefits and wide application prospect.