Abstract: From the angle of applications given by public telecom enterprises (China Telecom, China Unicom, etc.), the comparative research between traditional PSTN and VOIP has been done in this paper. The switching technology, signal system, protocols as well aw routines were compared and analyzed. The results show that VOIP were much better than PSTN in switching technology. Costing, band consuming, openness and flexibility. Based on the latest official Telecom costing files, the costing comparative results was given out between VOIP and PSTN. The interactive effecting model was given out in this paper. The opinion that VOIP and PSTN should be treated as a whole system was also pointed out.