Herein, we report a neutral iridium complex, [Ir(4-(2-pyridinyl)benzaldehyde)2(acetylacetone)] (Ir-ER), with viscosity-responsive phosphorescent emission intensity and lifetime. Quantitative measurement by two-photon phosphorescent lifetime imaging shows that the viscosity of ER increases significantly in the process of erastin-induced ferroptosis. Our work provides an effective strategy for quantitative measurement of the micro-environmental alternations of subcellular organelles during a specific cell death process.