The invention discloses a composite fiber, a composite board and a method for preparing the composite board. The composite fiber is composed of a core material and a shell, wherein the shell is encircled and formed around the core material; the melting temperature of the shell is lower than that of the core material; and the strength of the shell is higher than that of the core material. Accordingto the composite fiber, the composite board with high the characteristics such as impact strength, easy molding, low extension, light weight and high heat resistance simultaneously can be prepared, so that the requirement of sport protection is met.