The invention discloses a method for calculating longitudinal and torsional coupling rigidity of an internal combustion engine crankshaft system. The method comprises the following steps of measuringand recording related parameters of the internal combustion engine crankshaft system; determining a bearing constraint condition of the crankshaft system; building a longitudinal and torsional coupling rigidity analysis model; applying a unit torque to the neck of a primary shaft of a target crank, establishing a spatial statically indeterminate system balance equation, and establishing a deformation coordination condition at a redundant constraint; obtaining a complementary equation for converting the deformation coordination condition into solving of unknown redundant constraint force; in combination with the system balance equation, solving support reverse force at a bearing of the crankshaft system, constraint force at the fixed end of the target crank and a constraint moment; applyingunit axial action force to the neck of the primary shaft of the target crank and the neck of a primary shaft of an adjacent crank, and performing calculation to obtain axial displacement flexibilityunder the action of the unit torque; and finally obtaining coupling rigidity. The method is high in precision of calculating the longitudinal and torsional coupling rigidity of the internal combustionengine crankshaft system, and is easy for engineering realization.