Major changes including spiking crude oil samples and blank solutions with a known amount of stearic acid and replacing potassium hydroxide（KOH）with tetrabutyl ammonium hydroxide in titration were used to improve traditional measuring method of oil acid number，which had the defects that the oil with low acid number needed excess crude oil and did not have clear titration endpoint. The accuracy of modified titration method and national standard method was compared by testing the acid number of simulation oil and crude oil. The results showed that comparing national standard method，the modified titration method had higher accuracy，clearer titration endpoint，lower relative error and more evident peak value of first derivative differential curve，especially for the oil with low acid number and less sample amount.