The polymerization sequence of a benzoxazine monomer (A1) bearing alkynyl functional group is tuned by acetylacetone nickel (AN), and the resultant polybenzoxazines are used as precursors to prepare C/Ni composites with high electromagnetic interference shielding effectiveness (EMI SE). It is found that AN effectively promotes the trimerization of the alkynyl groups and forms more benzene rings in the obtained polybenzoxazines [poly(A1/AN)s], whereas polyA1 is a typical phenolic Mannich-type polybenzoxazine. Comparatively, poly(A1/AN)s possess higher cross-linking density, higher modulus at rubbery state and higher char yield at 800 degrees C. After carbonization, polyA1 heavily swells and deforms and the corresponding carbonaceous material cannot be used as EMI shielding material. Nevertheless, poly(A1/AN)s show good dimensional stability and the resultant magnetic carbonaceous material [C(A1/AN)s] possesses high electrical conductivity above 6 S/m and EMI SE above 25 dB. Thorough investigations reveal that appropriate amount of AN is beneficial to high graphitization degree, but excess AN lowers the graphitization degree. Therefore, the electrical conductivity decreases, while the magnetic hysteresis loss increases with the increasing of AN. Consequently, the EMI SE of C(A1/AN) increases and then decreases with the addition of AN, with C(A1/10AN) offering the highest EMI SE of 50 dB and showing an absorption-dominant mechanism.