Provided is use of a nano carbon sol as a fertilizer synergist, characterized in that, the nano carbon sol has a pH value of <2.2; a conductivity of 2800-3400μs/cm; a carbon mass percentage of 0.57-2%, a particle size of <100nm; and a potential difference of -30~35mV, and the adding amount of the nano carbon sol as a synergist in a nitrogen-containing fertilizer is 1.05-10% by weight of the nitrogen-containing fertilizer. The nano carbon sol may be added during the growth of a crop, or in the production process of the nitrogen-containing fertilizer. The invention enables smooth suppliment of the nutrients of the fertilizer, significantly improved yield, no shortage of the fertilizer during later growth stage of the crops, an average crop yield increasing rate of 5%-18%, and improved fertilizer utilization efficiency. The invention enables a nitrification inhibition rate of 6.3-6.8%, and a nitrification inhibition rate of the nitrogen-containing compound fertilizer of up to 19-25%, and thus the nano carbon so
