Abstract：The method of boriding by molten salt electrolysis was applied for TC4 titanium alloy to improve the surface hardness. Using a mixture of sodium tetraborate and sodium carbonate as the base ingredients of molten electrolyte, the experiment was carried out with the Voltage of 1.49V; the effects of boriding temperature on the microstructure of the boridng layer and the reaction mechanism were studied. DSC-TG analysis was conducted on the mixture of sodium tetraborate and sodium carbonate with different composition to analyze the changes when heating the mixed salts, XRD was used for chemical phase analysis of the molten salt and the borides coating, SEM and EDS were applied to observe microstructure and analyze the element content of the borides coating. It was found that a dense and uniform boriding layer was obtained on the TC4 titanium alloy with the main composition of TiB2 and TiB when the boriding temperature was 900℃ with process duration for 110min; the surface micro hardness of the boriding sample reached to 2.4GPa; NaBO2 was detected in the electrolyte after electrolysis; TiB2 was firstly produced in the bording layer then TiB was formed beneath it.