A method coupling the electromagnetic field with the circuit and the motion, which is used to calculate and analyze the transient thrust forces of a vertically reciprocate cylindrical linear induction motor, is given in the paper. The trust forces can be directly calculated by the Coulomb virtual work form the coupled equations excited by a 3-phase AC voltage sources. In the analysis, the lengthways end effects, relative motion between the primary and the secondary, and ferromagnetic material nonlinearities are all accounted at different initial voltage valves.