Abstract: Porosity is one of the important parameters in reservoir evaluation. Density logging is commonly used to determine the porosity of rock formations in petroleum exploration. With studies on basic principles, instrument and calibration method of density logging, the calculation formulas for porosity in different situations are derived. Such formulas are applied in the Matlab program to process the practical logging data and calculate porosities of rock formations in multiple well sections. The result shows that the method of density logging is highly applicable in porosity calculation.