A method for tin (Sn) film prepared by electroless plating on the surface of Cu substrate was realized, the silvery-white and semi-bright film has been obtained. The deposition mechanism is discussed and summed up period of replacement reaction, co-deposition period of Cu and Sn, and period of self-catalyzed deposition of Sn. The deposition process takes place because of the change of Cu and Sn potential when thiourea is added into the bath. That the film thickness increases linearly with the rise of plating times proves that the continuous self-catalyzed deposition of Sn has been realized, and the possibility of immersion plating is also excluded. The diffraction peak of Cu phase in the film becomes weaker and weaker, and that of Sn phase becomes stronger and stronger with increasing plating times. Additives B and C can play a role of grain refinement.