The invention relates to the technical field of welding, in particular to a preparation method for Sn based composite solder sheets. The preparation method comprises the following steps that S1, trepanned foam metal is subject to surface treatment; S2, filling of molten Sn based solder is carried out, specially, an infiltration manner is adopted for filling the foam metal with the molten Sn basedsolder, then foam metal sheets obtained after surface treatment are immersed into the molten Sn based solder, and the foam metal sheets are taken out after holes are filled with the solder; and S3, alternative stacking hot pressing treatment is carried out, specially, the foam metal sheets filled with the Sn based composite solder are alternatively stacked together, the stacked sheets are subjectto repeated hot pressing treatment, and finally the Sn based composite solder sheets are obtained. The preparation method has the beneficial effects that liquidity of the Sn based solder during brazing is guaranteed; the stacking layer number of foam Cu and foam Ni is changed, and the proportion of Cu to Ni in the composite solder sheets can be effectively controlled; and the strength and the hightemperature stability of a soldered joint are effectively improved.