Cyclotrimerization of alkynes to aromatics represents a promising approach to fabricate two-dimensional conjugated networks due to its single-reactant and atom-economy attributes,in comparison with other multicomponent coupling reactions.However,related reaction mechanism of alkyne cyclotrimerization has not been well understood due to technical challenges.In this work,we take a surface chemical reaction approach to study fundamental polymerization mechanism by using a diyne monomer named 4,4-diethynyl-1,1-biphenyl as a test bed.We have succeeded in directly characterizing reactant,intermediates and its reaction products by using scanning tunneling microscopy,which allows us to gain mechanistic insights into the reaction pathways.Our result reveals experimentally for the first time that the polycyclotrimerization is a two-step [2+2+2] cyclization reaction.This work provides an in-depth understanding of polycyclotrimerization process at the atomic level,offering a new avenue to design and construct of single-atom-thick conjugated networks.