Floorplanning is a critical step in the physical design of Very Large Scale Integrated (VLSI) circuits. Its main target is optimizing the layout area and interconnection wire length of chips, which can be transformed into a Multi-objective Optimization Problem (MOP). In this papers we propose a discrete Particle Swarm Optimization (PSO) algorithm for MOP which could take many key objectives into consideration and give a good compromise between them. The experiments on MCNC benchmarks show that the proposed algorithm is effective, and gives out many optional results for user's choice according to partialness, which can not be finished by traditional methods.