Flexible and transparent artificial synapses with extremely low energy consumption have the potential for brain-like neuromorphic electronics. However, most of transparent materials for flexible memristive artificial synapses were reported to show picojoule-scale high energy consumption with kiloohm-scale low resistance, which limits scalability of parallel operation. Here, we report on the flexible memristive artificial synapse based on Cs3Cu2I5 with energy consumption as low as 10.48 aJ (=10.48×10-18 J)/μm2 and resistance as high as 243 MΩ for writing pulses. Interface-type resistive switching at Schottky junction between p-type Cu3Cs2I5 and Au is verified, where migration of iodide vacancy and asymmetric carrier transport due to 3 times heavier effective mass of hole are found to play critical role in controlling conductance, leading to high resistance. Little difference in synaptic weight updates with high linearity and 250 number of states is observed before and after bending the flexible device. Moreover, the MNIST-based recognition rate over 90% is maintained upon bending, indicative of promising candidate for highly efficient flexible artificial synapses.