We investigate the electrical bias-induced degradation mechanism in non-encapsulated mixed halide perovskite CH₃NH₃PbI_3-xCl_x solar cells in ambient conditions using spatial and time-resolved electroluminescence (EL) measurements. The analyses of spatially resolved EL images reveal the inhomogeneous distribution of luminescence efficiency of the perovskite layer. A significant decrease of ~33% in power conversion efficiency (PCE) from 12.15 to 8.15% is found after applying a bias voltage larger than the open-circuit voltage (V_oc) for ~60 min in the ambient environment. Our findings show that the combined effects of cation defects (MA⁺/V_MA+) migration and decomposition of the perovskite layer are primarily responsible for degrading EL intensity and the photovoltaic properties.