The invention relates to a high speed railway subgrade compaction index lossless calculating method. On the premise that a built subgrade is not damaged, compaction indexes including K, K30, EVd and the like inside the subgrade are effectively obtained. The method includes the following steps that (1) heavy dynamic touch testing is developed in the center of a subgrade face layer in the vertical direction, and the hammering number N63.5 at different vertical depths H below the subgrade face layer are measured; (2) the subgrade compaction indexes including K, K30 and EVd are determined according to the following formula, and please see the specification for the formula; and (3) holes formed due to the heavy dynamic touch testing in the step (1) is subject to cement mortar or medium-coarse sand backfill compaction.