Lead　sulfide　colloidal　quantum　dots　(PbS　CQDs)　have　shown　great　potential　in　photodetectors　owing　to　their　promising　optical　properties,　especially　their　strong　and　tunable　absorption.　However,　the　limitation　of　the　specific　detectivity　(D*)　in　CQD　near-infrared　(NIR)　photodetectors　remains　unknown　due　to　the　ambiguous　noise　analysis.　Therefore,　a　dear　understanding　of　the　noise　current　is　critically　demanded.　Here,　we　elucidate　that　the　noise　current　is　the　predominant　factor　limiting　D*,　and　the　noise　is　highly　dependent　on　the　trap　densities　in　halide-passivated　PbS　films　and　the　carriers　injected　from　the　Schottky　contact　(EDT-passivated　PbS　films/metal).　It　is　found　that　the　thickness　of　CQDs　is　proportional　to　their　interface　trap　density,　while　it　is　inversely　proportional　to　their　minimal　bulk　trap　density.　A　balance　point　can　be　reached　at　a　certain　thickness　(136　nm)　to　minimize　the　trap　density,　giving　rise　to　the　improvement　of　D*.　Utilizing　thicker　PbS-EDT　films　broadens　the　width　of　the　tunneling　barrier　and　thereby　reduces　the　carrier　injection,　contributing　to　a　further　enhancement　of　D*.　The　limiting　factors　of　D*　determined　in　this　work　not　only　explain　the　physical　mechanism　of　the　influence　on　detection　sensitivity　but　also　give　guidance　to　the　design　of　high-performance　CQD　photodetectors.