[Objective] To understand the field of microbial oil displacement mechanism, evaluation standards and its influence factors. [Methods] The growth, migration and distribution law of microbes during microbial displacement oil were studied based on site tracing monitoring during field microbial displacement oil and the physical model experiment in laboratory. [Results] The results show that the exogenous microbes injected from water injection well can grow and reproduce well in oil reservoir, and the injected nutrient solution is also capable of activating indigenous microbes, but the microbes concentration in produced fluid is lower 1?2 orders of magnitude than injected microbes concentration due to the formation permeability and nutrient concentration. Because of the rapid glucose degradation and the microbes filtration and adsorption by formation, the amount of microbes stay near the wellbore and only part of them can be produced from the corresponding oil well. Moreover, the migration rate of microbes is slower than nutrition. [Conclusion] The formation permeability and the nutrient concentration of output liquid are the two key factors on microbial quantity. The microbes concentration in produced fluid is difficult to exceed 106 cells/mL on site. The results of the study have a great significance for the development and application of microbial displacement oil technology.