The invention pertaining to the field of convertor steelmaking technology provides a technique of converter smelting by using a dephosphorization agent. In a same convertor, dephosphorization is performed by using a CaO-base ephosphorization agent containing a few CaCl2 under favorable thermodynamics and combined-blowing dynamics conditions such as low temperature at the prior period, high content of iron oxide and the like, the prior period, desilication and dephosphorization slags of 50-70% are poured away in the molten iron preprocessing process, in the later period, decarburization process is performed and decarburization slag is formed and is then utilized as dephosphorization slag of the lower converter. According to the smelting technology, CaCl2 is added to the CaO-base ephosphorization agent to accelerate the burn-off rate of CaO by using the eutectic point character of CaCl2 and CaO binary system, the utilization ratio of lime is also improved, the utilizing of decarburization slag not only fully utilizes the physics heat brought by the decarburization slag, but also provides high content iron oxide for the dephosphorization of the prior period and accelerates the dephosphorization reaction.The invention has advantages of low cost of investment, shortened smelting period, carbon content of 0.4% at the end point of smelting, phosphorus content capable of being reduced to 0.006% and dephosphorization ratio up to 95.5%.