Abstract: By choosing a representative soft soil foundation in the Pear River Delta Area, sampling around piles within a certain distance and analyzing their microstructure, the radius of reinforcement with gravel piles was discussed. At first, getting soft soil samples in their natural state and consolidation state after 45 days from the position located at 1d to 6d from piles (where d is the diameter of the gravel pile), obtaining and analyzing their microstructures by scanning electronic microscope, parameters describing pores and grains were extracted in deferent states. Then, by comparing the features of microstructures and the variation of parameters in different positions, the key point at 3d was gained, meaning that the microstructure would vary greatly within a distance of 3d (the porosities of soils around reinforcement were all larger than 50%), and then changeed very small beyond 3d (the porosities of soils around reinforcement were all less than 50%, mainly distributing in the range of 50%～ 10%). According to the result, a conclusion is obtaind that an effective radius of gravel piles in soft clays of Pearl River Delta is 3d. It provides a important reference for the reinforcement design of gravel pile.