Provided are an electrical tree test device for a silicone rubber material for a cable accessory and a method for preparing a sample. The method includes: adding a semi-conductive silicone rubber into xylene, performing spraying on a surface of a silicone rubber insulation sample sheet, performing a curing processing, cutting the sample sheet into a high-voltage electrode with a triangular longitudinal section end, then adhering the high-voltage electrode on the surface of the silicone rubber insulation sample sheet, and performing a high temperature vulcanization to obtain a silicon rubber sample sheet; placing the silicon rubber sample sheet into a mold, injecting a high temperature vulcanizable liquid silicone rubber mixture, and performing the high temperature vulcanization, to obtain a sample; and performing cutting at a position distanced from a tip of the high-voltage electrode by 2 mm, and adhering a ground electrode of a flat plate-like structure at the cross section.