The invention belongs to the stainless steel surface processing field and relates to a stainless steel electrochemical polishing solution and its use method. The polishing solution is composed of a mixed acid and a composite additive, the mixed acid is an aqueous solution composed of phosphoric acid and phytic acid, and the mass ratio of water to phytic acid to phosphoric acid is 1:1:10-1:1:5; the composite additive is an aqueous solution composed of glycerin, saccharin and phosphoric acid, and the volume concentrations of glycerin, saccharin and phosphoric acid are 200-300g/L, 50-60g/L and 50-100g/L respectively; and the electrochemical polishing solution is obtained through adding 30-50mL of the composite additive to 1000mL of the mixed acid solution, and uniformly stirring. Polishing technological parameters comprise a temperature of 50-80DEG C, a voltage of 8-20V, a current density of 0.5-80A/dm<2> and an average polishing time of 0.5-30min/dm<2> when the electrochemical polishing solution is used. The polishing solution has the advantages of large current density range, suitableness for different materials of the stainless steel, simple technology, convenient operation, high polishing efficiency, realization of the continuous electrochemical polishing treatment of the inner and outer walls of large stainless steel devices, less application amount, and small power supply device load.