The invention provides a method for applying dependent variables to calculate porosity of a brittle formation. The method comprises following steps: firstly, making structural interpretations about brittle formations and tomography; applying interpretation data to set up a geologic model in a depth domain; adopting a three-dimensional structural recovery method to calculate accumulative dependent variables of the brittle formation; utilizing a drilling and coring test to solve rock elasticity parameters of the brittle formation; calculate porosity of the brittle formation based on the set dependent variables-rock elasticity parameters-fracture porosity relations; applying core porosity to make corrections and compiling a horizontal distribution graph of porosity. Method for applying dependent variables to calculate porosity of the brittle formation helps to increase accuracy of porosity of the brittle formation and is easily and conveniently used and is well applied to increase of success rate of oil and gas reservoir, drilling and exploitation, avoidance of construction risks and improvement of efficiency.