A carbon-based auxiliary, a preparation method thereof and application in livestock manure composting are characterized in that the carbon-based auxiliary is a granular biomass homogeneous body which is obtained through incomplete splitting decomposition under an oxygen-free condition at a temperature of 650 DEG C to 750 DEG C, has granularity of 5 cm to 13 cm and is not completely carbonized. The application includes the following steps of firstly, mixing livestock manure, conventional auxiliary and the carbon-based auxiliary according to the volume ratio of 1:(0.33-0.5):(0.5-0.67) to obtain a first mixed material; secondly, conducting composting through a high-temperature aerobic fermentation mode, turning piles, and discharging materials 28-35 days later; thirdly, screening the carbon-based auxiliary out of the piles, air-drying the carbon-based auxiliary, and making the air-dried carbon-based auxiliary serve as a recycled carbon-based auxiliary; fourthly, when processing the next batch of piles, mixing the livestock manure, the conventional auxiliary, the recycled carbon-based auxiliary and the primarily-used carbon-based auxiliary according to the volume ratio of 1:(0.33-0.5):(0.25-0.34):(0.25-0.33) to obtain a second mixed material, and conducting composting. The effects on reducing the consumption of the conventional auxiliary, promoting temperature rise, shortening the cycle, recycling the carbon-based auxiliary and the like are achieved.