Semiconductor mixed-halide perovskites featured with a tunable energy bandgap are ideal candidates for light absorbers in tandem solar cells as well as fluorescent materials in light-emitting diodes and nanoscale lasers. These device advancements are currently hindered by the light-induced phase segregation effect, whereby ion migration would yield smaller-bandgap domains with red-shifted photoluminescence. Here we show that upon laser excitation all-inorganic mixed-halide nanocrystals unexpectedly exhibit a blue shift in the photoluminescence peak that can revert back in the dark, thus depicting the processes of ion migration out of and back to the originally excited nanocrystals. Interestingly, this reversible photoluminescence shift can also be induced by electrical biasing of mixed-halide nanocrystals without the injection of charge carriers. The above findings suggest that it is the local electric field that breaks the ionic bonds in mixed-halide nanocrystals, which could be a universal origin for light-induced phase segregation observed in other mixed-halide perovskite materials.