The removal of a sapphire substrate by laser lift-off, photoluminescence detection technology, and the luminous efficiency of size-dependent devices are very hot issues for the Micro-LED display, which is thoroughly studied in this paper. The mechanism of thermal decomposition of the organic adhesive layer after laser irradiation is analyzed in detail, and the thermal decomposition temperature of 450 °C solved by the established one-dimensional model is highly consistent with the inherent decomposition temperature of the PI material. The spectral intensity of PL is higher, and the peak wavelength is red-shifted by about 2 nm compared to EL under the same excitation condition. The results of size-dependent device optical-electric characteristics show that the smaller the device size, the lower the luminous efficiency under the same display resolution and PPI conditions, and the higher corresponding display power consumption.