Based on the proposed optical readout uncooled infrared (IR) imaging system，several batches of the substrate-free focal plane array (FPA) with different pitches have been successively fabricated，and by using these fabricated FPAs，the thermal images were obtained at room temperature. However，the theoretical analysis indicates that the deviation between the theoretical model assuming constant temperature of the supporting frames of the FPAs and the experimental results gradually increases with the decreasing pitch size of from 200μm to 60μm. Based on the finite element method，the thermal characteristic of those FPAs was simulated and the reason for the deviation is found to be that the temperature-constant assumption is not quite appropriate for the substrate-free FPA，and the absorption of the thermal radiation will obviously result in the temperature rise of the supporting frame，which would increase the temperature of the adjacent pixels. The imaging performance of the substrate-free FPA is also analyzed. Compared with the substrate FPA with the same pitch, the thermal detection sensitivity of the substrate-free FPA with 60μm pitch increases by about one order of magnitude.