The hydrogen-transfer reaction of W + NH3 incorporates four possible diabatic reaction pathways along with septet, quintet, triplet, and singlet states. The intersystem crossings thus play an important role in the reaction mechanisms. In this work, ab initio and DFT methods are used to determine all possible intermediates, transition states, products, and intersystem crossing points as well as the spin orbit couplings. The mechanism of hydrogen elimination is further revealed by the natural bond orbital analysis. From the rate constants yielded by a nonadiabatic transition state theory, we find that two intersystem crossings significantly change the reaction pathways. Finally, we suggest a feasible reaction pathway with exothermicity 72.8 kcal/mol, which is consistent with the experimental measurements.