Effects of insecticide acetamiprid on photosystem II (PSII) activity of Synechocystis sp. were investigated by a variety of in vivo chlorophyll fluorescence tests. Acetamiprid exposure increased the proportion of inactivated PSII reactive centers (PSIIX) and led to loss of active centers (PSIIA). High concentration (1.0 mM) acetamiprid decreased amplitude of the fast phase and increased the slow phase of fluorescence decay during Q A reoxidation. The electron transport after QA was hindered by high concentration acetamiprid and more QA had to be reoxidized through S2(QAQB) charge recombination. Acetamiprid decreased the density of the active reaction centers, electron transport flux per cross section and the performance of PSII activity but had little effect on dissipated energy flux per reaction center, antenna size and the maximum quantum yield for primary photochemistry (Fv/Fm). The target site of acetamiprid toxicity to the PSII of Synechocystis sp. was electron transfer on the acceptor side.