The invention relates to a preparation method of endo-tetrahydro-dicyclopentadiene. The method of preparing endo-tetrahydro-dicyclopentadiene with ethylene byproduct C9 is that: the ethylene byproduct C9 is filled into a hydrogenator 1 that is filled with catalyst; hydrogenation is carried out under warming and pressuring condition and with the function of catalyst; hydrogenated materials that come out of the hydrogenator 1 enter a second hydrogenator that is filled with catalyst, and hydrogenation is carried out under the warming and pressuring condition and the function of catalyst; hydrogenated materials that come out of the hydrogenator 2 enter a rectifying tower 1, and distillate of 120 DEG C to 190 DEG C that is produced under normal pressure distillation is acquired from the top of the tower; materials that come out of the rectifying tower 1 enters a rectifying tower 2, and distillate of 190 DEG C to 192 DEG C that is produced under normal pressure distillation is acquired from the top of the tower, and the distillate is the endo-tetrahydro-dicyclopentadiene product with the purity higher than 97 percent.