A　simulation　algorithm　based　on　the　law　of　large　number　and　Monte　Carlo　method　in　the　classical　computer　environment　is　presented.　During　the　simulation　of　the　original　quantum　BB84　protocol　in　ideal　environment,　the　sender　Alice　tries　to　send　classical　bit　0　or　1　to　the　receiver　Bob,　and　the　eavesdropper　Eve　tries　to　get　the　transmission　information　by　intercepting　and　resending　the　quantum　particles.　The　bit　error　rate　in　the　quantum　BB84　protocol　is　also　given,　and　the　value　of　the　bit　error　rate　can　be　analyzed　if　Eve　eavesdrops　the　communication.　In　addition,　the　mean　square　error　is　introduced　to　describe　the　similarity　between　the　simulation　data　and　the　theoretical　data.　(The　smaller　the　mean　square　error　is,　the　more　reasonable　the　simulation　will　be.)　In　this　simulation,　the　value　of　MSE　is　6.705x10-5　after　5000　times　simulation　when　Eve　eavesdrops　the　communication　with　the　probability　of　100%.　The　time　complexity　of　the　simulation　algorithm　is　O(n)　in　our　experiment.　The　reason　why　there　is　always　an　error　between　the　simulation　data　and　theoretical　data　is　analyzed,　and　the　correctness　and　rationality　of　the　simulation　algorithm　are　also　analyzed.