The invention discloses a preparation method of an electric vehicle battery pack box body upper cover, and relates to the technical field of an automobile vehicle body part. The preparation method comprises the following steps of mixing carbon fiber, compatibilizers, polylactic acids, fire retardants, plasticizers and antioxidants according to a mass ratio of 1:(0.015 to 0.675):(0.65 to 8.048):(0.045 to 1.182):(0.045 to 1.182):(0.001 to 0.131) to prepare a mixed material; performing melt blending on the mixed material at the temperature being 140 DEG C to 200 DEG C to obtain a carbon fiber enhanced polylactic acid composite material; performing injection forming on the carbon fiber enhanced polylactic acid composite material to obtain a battery box body upper cover. The carbon fiber enhanced polylactic acid composite material is used for preparing the light-weight upper cover; by using the polylactic acid, the box body upper cover provided by the embodiment has biodegradable performance; the environment pollution of the upper cover after the abandon can be avoided. The carbon fiber is added into the polylactic acid, so that the carbon fiber enhanced polylactic acid composite material is formed for preparing the box body upper cover; the physical performance of the box body upper cover is ensured.