This article compares several CO2 miscibility pressure predicting methods via slim tube tests, modifies the method of National Petroleum Council, and builds the method fitting for China's oilfield to predict the minimum miscibility pressure of CO2 flooding, which laid the foundation for the phase changing study. In addition, a set of visible pressure volume temperature device, computed tomography device, and scanning electron microscope device is induced to show the phase changing of miscible and immiscible flooding under the condition of different injection volumes and different injection rates, which may provide visualized test results to support the reliability of the critical miscibility pressure predicting method. Such a method is a revolution of tiny flow testing experiments in porous media, and has lent a way to a deeper investigation on the unique phase changing mechanism of China's continental oil.