The　traditional　power　spectrum　density　peak　method　was　improved　according　to　the　feature　of　dense　mode　shapes　of　spatial　lattice　structures.　The　union　set,　combining　the　average　normalized　power　spectrum　measured　at　all　the　test　points　with　an　auxiliary　normalized　power　spectrum　calculated　according　to　the　characteristics　of　theoretical　mode　shapes,　was　defined　as　the　modal　identification　results.　Besides,　the　judgment　criterion　was　discussed.　The　improved　power　spectrum　density　peak　method　can　avoid　the　missing　of　modes　and　sieve　the　duplicated　frequencies　in　identification　process.　A　numerical　example　shows　the　effectiveness　of　the　method　improved.　A　software　was　made　based　on　the　above　theory　and　has　been　applied　to　the　monitoring　system　of　the　stadium　for　Shenzhen　Universiade.