The invention relates to a preparation method of a porous carbon nanosphere with a controllable diameter. In terms of weight, phloroglucinol, terephthalaldehyde and water are taken in the proportion of 1:0.7-0.9:300-340. A phenolic prepolymer solution is obtained after being reacted for 15 minutes to 12 hours at the stirring speed of 400-1000 revolutions per minute at the temperature of 70 DEG C. In terms of weight, resorcinol, formaldehyde solution and ammonia water are taken in the proportion of 1:1.5-1.8:0.4-0.45, and mixed. A mixed solution is obtained. The mixed solution is added dropwisely into the phenolic prepolymer solution. The mass proportion of the mixed solution and the Phenolic prepolymer solution is 1:40-45. Obtained solution is reacted for 24 hours at the stirring speed of 400-1000 revolutions per minute and undergoes hydro-thermal treatment under the temperature of 100 DEG C, and polymer nanospheres are obtained after an obtained product is dried for 2-12 hours under the temperature of 40-100 DEG C. The polymer nanospheres are protected by inert gas in a tube furnace and heated to 600-1000 DEG C for carbonization at the heating rate of 0.5-10 DEG C per minute. The porous carbon nanaspheres can be obtained when the temperature of the polymer nanospheres naturally lowers to the room temperature. The porous carbon nanosphere is controllable in diameter, good in dispersity, and improtant in application prospect in aspects such as catalysis, environment, energy and biology.