The invention relates to a stainless steel surface treatment reagent, in particular to a stainless steel surface colouring method. According to the invention, with water as a solvent, each litre of solution contains 30-80g of molybdate, 10-18g of boric acid, 2-5g of zinc sulphate, 0.5-5g of manganese sulphate, 2-15g of antimony nitrite, 0.2-2g of lanthanum nitrite and 0.1-10g of sodium malonate. The stainless steel surface colouring method comprises the following basic flow: oil removing, electrolytic polishing, cleaning, colouring, cleaning and drying and obtaining the finished product, wherein an anode is a workpiece to be coloured, a cathode is a lead plate, a colouring current is controlled to be 0.1-5A / dm<2>, voltage is 1.5-2.5V, a colouring temperature is room temperature, colouring time is 0.1-30 minutes, cleaning and drying are carried out after colouring, and the surface of the obtained sample is uniform and bright. The stainless steel electrochemical colouring process provided by the invention needs simple equipment, is easy to operate, contains no toxic substance and is beneficial to environmental protection; and colouring is stable, and a high-quality colour stainless steel product can be produced by adopting the stainless steel electrochemical colouring process.