Simultaneously extended pi-conjugated system and provide abundant pore structure of semiconductor photocatalysts for hydrogen (H-2) production is highly desirable. Hence, a novel mesoporous sulfurized polyacrylonitrile modified g-C3N4 (g-C3N4/S-PAN) pi-conjugation heterojunction is firstly fabricated by one-step strategy under the sulfur-induced cyclization reaction and pore-creating effect. Excitedly, the g-C3N4/S-PAN pi-conjugation heterojunction extends the n-conjugated system in favor of speeding up the photo generated electron transfer, which is due to strengthen the pi-pi interactions between the SPAN and g-C3N4 and S-PAN is more apt to accept electrons. And the obtained g-C3N4/S-PAN Tc-conjugation heterojunction with mesoporous structure also provide abundant activesites for proton reduction. Accordingly, the g-C3N4/S-PAN-2 pi-conjugation heterojunction shows the optimal photocatalytic H-2 evolution (PHE) activity (736.24 gmol h(-1)g(-1)), which is approximately 2.15 times higher than pristine g-C3N4. In addition, the relationships of the optical and photoelectrochemical properties with photocatalytic activity are revealed in depth based on the first-principles calculations of band structure and density of states (DOS). This work provides a new one-step strategy to obtain g-C3N4-based pi-conjugation heterojunction with the unique microstructure for improving PHE activity. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.