A method for preparing a carbon material, comprising: a carbonization step in which a straight-chain polymer material containing nanocrystals is carbonized by using a strong oxidant, thus obtaining a carbon nanomaterial, wherein: the straight-chain polymer material containing nanocrystals means that the straight-chain polymer material has a crystalline portion and the crystal size of the crystalline portion is 1 nanometer to less than or equal to 100 nanometers; the carbon nanomaterial means the carbon powder composing the carbon material has a size of 1 nanometer to less than or equal to 100 nanometers; the straight-chain polymer material containing nanocrystals is polyolefins, polyacetylenes, or other straight-chain hydrocarbon polymeric materials with a symmetric structure; the strong oxidant comprises one or more of the followings: a compound of a general formula R-SO 3 H, wherein R is selected from F, Cl and Br; a combination of H 2 SO 4 and HCl; a combination of H 2 SO 4 and HF; concentrated sulfuric acid; fuming sulfuric acid; and a combination thereof; the carbonization step is carried out at a temperature which is from 1 to 50 °C lower than the crystallization temperature of the crystalline portion in the polymer material; the weight ratio of the polymer material and the strong oxidant is from 1:1 to 1:300. a carbonization step in which a straight-chain polymer material containing nanocrystals is carbonized by using a strong oxidant, thus obtaining a carbon nanomaterial, wherein: the straight-chain polymer material containing nanocrystals means that the straight-chain polymer material has a crystalline portion and the crystal size of the crystalline portion is 1 nanometer to less than or equal to 100 nanometers; the carbon nanomaterial means the carbon powder composing the carbon material has a size of 1 nanometer to less than or equal to 100 nanometers; the straight-chain polymer material containing nanocrystals is polyolefins, polyacetylenes, or other straight-chain hydrocarbon polymeric materials with a symmetric structure; the strong oxidant comprises one or more of the followings: a compound of a general formula R-SO 3 H, wherein R is selected from F, Cl and Br; a combination of H 2 SO 4 and HCl; a combination of H 2 SO 4 and HF; concentrated sulfuric acid; fuming sulfuric acid; and a combination thereof; the carbonization step is carried out at a temperature which is from 1 to 50 °C lower than the crystallization temperature of the crystalline portion in the polymer material; the weight ratio of the polymer material and the strong oxidant is from 1:1 to 1:300. the straight-chain polymer material containing nanocrystals means that the straight-chain polymer material has a crystalline portion and the crystal size of the crystalline portion is 1 nanometer to less than or equal to 100 nanometers; the carbon nanomaterial means the carbon powder composing the carbon material has a size of 1 nanometer to less than or equal to 100 nanometers; the straight-chain polymer material containing nanocrystals is polyolefins, polyacetylenes, or other straight-chain hydrocarbon polymeric materials with a symmetric structure; the strong oxidant comprises one or more of the followings: a compound of a general formula R-SO 3 H, wherein R is selected from F, Cl and Br; a combination of H 2 SO 4 and HCl; a combination of H 2 SO 4 and HF; concentrated sulfuric acid; fuming sulfuric acid; and a combination thereof; the carbonization step is carried out at a temperature which is from 1 to 50 °C lower than the crystallization temperature of the crystalline portion in the polymer material; the weight ratio of the polymer material and the strong oxidant is from 1:1 to 1:300.