Detection　of　haloalkanes　is　of　great　industrial　and　scientific　importance　because　some　haloalkanes　are　found　serious　biological　and　atmospheric　issues.　The　development　of　a　flexible,　wearable　sensing　device　for　haloalkane　assays　is　highly　desired.　Here,　we　develop　a　paper-based　microfluidic　sensor　to　achieve　low-cost,　high-throughput,　and　convenient　detection　of　haloalkanes　using　perovskite　nanocrystals　as　a　nanoprobe　through　anion　exchanging.　We　demonstrate　that　the　CsPbX3　(X　=　Cl,　Br,　or　I)　nanocrystals　are　selectively　and　sensitively　in　response　to　haloalkanes　(CH2Cl2,　CH2Br2),　and　their　concentrations　can　be　determined　as　a　function　of　photoluminescence　spectral　shifts　of　perovskite　nanocrystals.　In　particular,　an　addition　of　nucleophilic　trialkyl　phosphines　(TOP)　or　a　UV-photon-induced　electron　transfer　from　CsPbX3　nanocrystals　is　responsible　for　achieving　fast　sensing　of　haloalkanes.　We　further　fabricate　a　paper-based　multichannel　microfluidic　sensor　to　implement　fast　colorimetric　assays　of　CH2Cl2　and　CH2Br2.　We　also　demonstrate　a　direct　experimental　observation　on　chemical　kinetics　of　anion　exchanging　in　lead-halide　perovskite　nanocrystals　using　a　slow　solvent　diffusion　strategy.　Our　studies　may　offer　an　opportunity　to　develop　flexible,　wearable　microfluidic　sensors　for　haloalkane　sensing,　and　advance　the　in-depth　fundamental　understanding　of　the　physical　origin　of　anion-exchanged　nanocrystals.