The purpose of the invention is to provide a method and a system for monitoring the void under the backfill subgrade and reducing the settlement by grouting under the subgrade, so as to solve the problems of cracking caused by excessive deflection of the subgrade structure caused by void and driving safety caused by uneven settlement of the subgrade. The main technical features are as follows: according to the subgrade settlement monitoring to find the appropriate grouting timing, through the reserved grouting hole combined with porous simultaneous grouting construction technology to quickly complete the subgrade grouting settlement reduction.