NOVELTY - A hydrocarbon fuel preparation method involves performing a thermal conversion process by adding catalyst and water to plant oil in a sealed reaction kettle, stirring and mixing the obtained mixture, and heating and naturally cooling the mixture. The obtained product is heated, dehydrated and filtered. The filtered product is heated, added with a decarboxylation catalyst i.e. DDL type catalyst, and subjected to decarboxylation cracking process to obtain crude refining hydrocarbon compound fuel. The fuel is subjected to a normal pressure distillation process to obtain a finished product.    USE - Method for preparing hydrocarbon fuel from plant oil.    ADVANTAGE - The method enables greatly improving thermal performance of the fuel by controlling the reaction temperature and the type of catalyst, and significantly increasing the yield of alkane content of the fuel.    DETAILED DESCRIPTION - A hydrocarbon fuel preparation method involves performing a thermal conversion process by adding catalyst and water to plant oil in a sealed reaction kettle, uniformly stirring and mixing the obtained mixture, heating the mixture at 280-550 degrees C with heating rate of 10-80 degrees C/minute, and naturally cooling the mixture at room temperature for 5-30 minutes, where adding amount of the catalyst is 1-10 wt.% of plant oil, adding amount of water is 2-40 wt.% of plant oil, and the catalyst is selected from transition metal catalyst or molecular sieve catalyst. The obtained product is heated at 60-80 degrees C, dehydrated at pressure of less than 0.05MP until the water content reaches less than 2% and filtered by 3 mm mesh filter to remove impurities. The filtered product is heated at 220-550 degrees C, added with a decarboxylation catalyst i.e. DDL type catalyst, and subjected to decarboxylation cracking process for 5-60 min to obtain crude refining hydrocarbon compound fuel. The fuel is subjected to a normal pressure distillation process at 60-500 degrees C to obtain a finished product with main components of alkane, olefin and aromatic hydrocarbon.