Four-dimensional dynamic simulation should be the principal method of goaf simulation and also the essential approach to understand laws of fields in the goaf. The exact determination of goaf porosity is a key problem for this method. In this research, the porosity of goaf is studied in three-dimensional space. At first, the porosity is regarded as a second-order tensor, and the computational method for the porosity in all directions is obtained. Secondly, porosity in the vertical direction is analyzed and the relationship between porosity and rock diameter is obtained through simulation test. The process curve is drawn and is proved to be correct by means of field observation and theoretical analysis. Finally, the air flow field of goaf in vertical direction is analyzed through numerical simulation, which in turn demonstrates the necessity to research on the goaf in three dimensions.