The influences of small dose B doping and heat treatment on the phase structure and magnetocaloric properties of La-Fe-Si melt spun ribbons were investigated. The results showed that the doping of B promoted the formation of the La(Fe,Si)13 phase. And La-Fe-Si-B alloys have already acquired more La(Fe,Si)13 phase before heat treatment compared to La-Fe-Si alloys. During the solidification process, it has the most content of the phase of La(Fe,Si)13 when the doping content is 0.08. B doping is beneficial for the refining of microstructure during rapid solidification which shortens the element diffusion path of peritectoid reaction in the following process of heat treatment(α-Fe + LaFeSi → La(Fe, Si)13)and is in favor of the formation of La(Fe,Si)13 phase. In the process of heat treatment, the microstructures of La-Fe-Si alloys were optimized by B doping compared to La-Fe-Si alloys. B doping refines the heat treatment structure of La-Fe-Si alloys and its Curie temperature and magnetocaloric effect has been improved. The Curie temperature of the ribbons changes from 210 K to 233 K，the maximum isothermal magnet entropy change promotes from 5.47 J/kg?K to 9.40 J/kg?K.