Highly efficient light-harvesting systems with the sequential energy transfer process are significant for utilizing solar energy in photosynthesis. Herein, we report a quadrilateral platinum(II) metallacycle containing tetraphenylethylene (M1) as a light-harvesting platform. The M1 assembly serves as an ideal donor because of the aggregation-induced emission (AIE) effect, realizing two-step sequential energy transfer from the M1 assembly to eosin Y (ESY) and then to sulforhodamine (SR101) with high efficiency. ESY was used as a bridge in a relay mode during this process. To better mimic natural photosynthesis, the M1-ESY-SR101 system was utilized as photochemical catalysis for alkylation of C-H bonds in aqueous solution, showing enhanced catalytic activity as compared with the M1-ESY system or ESY/SR101 alone.