The invention discloses a preparation method of bagasse nano-crystalline cellulose and a rubber composite material of the bagasse nano-crystalline cellulose. The method comprises the following steps: carrying out a stirring reaction on bagasse and a NaOH water solution; carrying out oxidation bleaching in a NaClO solution with the assistance of ultrasonic waves; stirring the mixture with a sulfuric acid solution for acid hydrolysis; adjusting the pH to 6-7, and carrying out ultrasonic treatment to obtain a bagasse nano-crystalline cellulose suspension liquid; stirring natural rubber latex for 1-30 minutes; adding nano-crystalline cellulose into the natural rubber latex, stirring and mixing uniformly the mixture; drying the mixture to a constant weight so as to obtain a solid bagasse nano-crystalline cellulose/natural rubber mixture; and mixing the solid mixture with carbon black so as to obtain the bagasse nano-crystalline cellulose and rubber composite material. After the bagasse nano-crystalline partially takes place of the carbon black, vulcanized rubber can reduce compression fatigue temperature rise while the performance of the natural rubber reinforced by the carbon black is fundamentally maintained, and a bagasse nano-crystalline/carbon black/rubber composite material can be prepared by adopting a traditional rubber processing technology. The composite material prepared by the invention can be applied to manufacturing various vulcanized rubber products such as rubber hoses and tires.