A flame-retardant three-dimension-molded object and a manufacturing method thereof are disclosed. The manufacturing method includes steps of: (a) providing a molded body including plural first pores and plural second pores, wherein the plural first pores communicate inward from an outer surface of the molded body, the first pores and the second pores are partially connected to each other, and a first average pore width of the first pores is greater than a second average pore width of the second pores; (b) immersing the molded body in a flame-retardant solution composed of a flame retardant and a solvent for an immersion time under a negative pressure, so that the flame-retardant solution is introduced into the plural first pores and the plural of second pores; and (c) removing the solvent contained in the plurality of first pores and the plurality of second pores, so as to obtain the flame-retardant three-dimensional-molded object.