In the technologies of supercritical CO2 flooding enhanced oil recovery，the development of assistant which can effectively reduce minimum miscibility pressure（MMP）is significantly important. In order to shorten development period and raise efficiency remarkably，a convenient and visualized method to measure MMP，namely rising height test，was introduced. A high pressure cell with transparent windows was set up and by transforming volume expansion of oil phase into height data，a miscible-ratio curve against equilibrium pressure would be obtained after data processing. The whole process could be visualized. The measurement results by this method were compared to those by slim tube test to verify test accuracy. An experiment method which utilized simulation oil samples consisted of different ingredients was introduced for assistant screening. The results showed that the MMP measured by this method was accurate and the relative error was less than 5% compared with that of slim tube test. With this estimation method，an effective octaester assistant was screened out，which was able to reduce the MMP of crude-oil samples from Changqing，Xinjiang and Jilin oilfield under the temperature of 50—80℃ and dosage of 1% —3% . The biggest decline was up to 20%. The MMP could be determined accurately by rising height test and the effect of miscible flooding assistant could be estimated.