The　Viterbi　algorithm　(VA)　decoding　is　a　kind　of　efficient　maximum-likelihood　(ML)　decoding　method　for　finite　state　signals　observed　in　noise　environments,　it　has　been　widely　used　in　digital　communication　systems.　The　constrained　Viterbi　algorithm　(C-VA)　makes　full　use　of　the　information.　of　the　known　correctly　decoded　bits　to　improve　the　performance.　In　this　paper,　we　focus　on　the　C-VA　algorithm　and　compare　it　with　the　conventional　VA.　Simulation　results　show　that　the　decoding　performance　has　been　greatly　improved　while　using　C-VA　in　additive　white　Gaussian　noise　(AWGN)　and　binary　symmetric　channel　(BSC).