Fluorescence　is　generally　adopted　in　molecular　diagnosis.　In　order　to　detect　weak　fluorescence　signal,　a　weak　fluorescence　detection　circuit　has　been　design　based　on　a　photomultiplier　tube.　The　circuit　mainly　consists　of　three　parts　which　are　a　photoelectric　converter　circuit,　a　pre-amplifier　circuit　and　a　filter　circuit.　This　circuit　is　able　to　receive　weak　fluorescent　signals　and　convert　them　into　electrical　signals　effectively.　As　a　result,　important　information　such　as　the　concentration　of　corresponding　reagnet　can　be　obtained　by　analyzing　the　intensity　of　fluorescence　received.　It　is　experimentally　demonstrated　that　the　designed　circuit　can　be　used　in　weak　fluorescence　detecting　efficiently.　Its　detection　limitation　is　as　low　as　the　fluorescent　signal　emitted　from　quantum　dots　of　10(-10)mol/ml,　making　it　suitable　for　low　fluorescence　detection　in　molecular　diagnosis.