The　security　of　an　MH　plaintext　is　intended　to　be　based　on　the　subset　sum　problem　(SSP).　However,　when　the　density　of　a　knapsack　is　less　than　0.9408,　SSP　can　be　solved　in　polynomial　time　through　the　LLL　lattice　basis　reduction　algorithm　for　seeking　the　shortest　non-zero　vector　in　a　lattice.　In　this　paper,　we　discuss　the　method　of　attack　on　the　MH　cryptosystem,　and　analyze　the　effect　of　the　length　and　density　of　a　knapsack　on　the　success　rate　of　the　LLL　lattice　basis　reduction.　In　theory,　when　the　density　is　less　than　0.9408,　the　success　rate　is　high.　From　the　data　we　get　by　experiments,　we　observe　that　when　the　density　is　fixed　and　the　length　is　increased,　the　success　rate　decreases,　and　when　the　length　is　fixed　and　the　density　is　increased,　the　success　rate　also　decreases.　Concretely,　when　the　length　is　80　and　the　density　is　0.8,　the　success　rate　is　closed　to　50%,　when　the　density　is　increased　to　0.952　and　the　length　remains　unchanged,　the　rate　is　less　than　50%,　and　when　the　density　is　increased　to　1.509　and　the　length　remains　unchanged,　the　rate　decreases　to　35%.　©　2011　IEEE.