In　recent　years,　the　development　of　biological　computers　is　becoming　faster　and　faster,　in　order　to　make　the　logical　operation　algorithms　of　biological　computers　more　mature　and　stable,　a　new　idea　for　the　code　lock　logic　circuit　is　proposed　based　on　DNA　strand　displacement　by　using　the　dual-rail　method.　The　code　lock　is　designed　by　four　input　signals　and　one　conversion　input　signal.　Only　when　the　four　input　codes　are　correct　and　the　conversion　signal　code　is　turned　on,　the　password　lock　will　be　in　open　state,　otherwise　the　password　lock　will　produce　an　alarm　signal,　stopping　outside　invasion　timely.　The　information　of　key　is　processed　to　obtain　the　correct　password;　finally,　the　experimental　simulation　results　are　obtained　by　Visual　DSD　software.　The　results　analysis　show　that　the　designed　code　lock　circuit　is　effective,　which　may　provide　a　good　technical　support　and　a　good　theoretical　basis　in　biological　computers　development.　©　2019　World　Scientific　Publishing　Company.