The invention provides a method of preparing electrode material for a carbon-based supercapacitor by taking bamboo powder as raw material. The method includes the following steps: under the protection of nitrogen, evenly mixing a carbonized product with alkali and water after bamboo powder as raw material undergoes two-stage carbonization, heating the above mixture to enable the water content of the mixture to be 10%-30% of that of the original mixture, conducting two-stage activation, cleaning and drying the mixture to obtain alkali activated bamboo powdered carbon, adding the alkali activated bamboo powdered carbon into a phosphor and nitrogen doped solution, conducting ultrasonic uniform mixing, then conducting a hydro-thermal reaction, and finally obtaining a finished product after post-treatment is conducted on a reaction solution. Cheap, environment-friendly and renewable bamboo that is easily available is taken as raw material to prepare carbon material for a supercapacitor, so the method has the noticeable cost advantage. Through two-stage carbonization and two-stage activation, impurities in bamboo material can be fully removed, and porous carbon material with high carbon content can be formed. Moreover, a phosphor, nitrogen and hetero atom codope process is introduced to enable bamboo carbon to have extra fake capacitance, so the capacitive performance of the supercapacitor can be greatly improved.