Based on thermal deformation of bi-material microcantilever, the focal plane array (FPA)of uncooled optical readout infrared(IR) imaging system has undergone a development from substrate array to substrate-free array. The experimental imaging result and finite element method (FEM) analysis indicated that the substrate-free focal plane array (FPA) did not accord with the condition of constant frame temperature. This paper proposed a new theoretical model on thermal transmission of substrate-free FPA with electrical analogy method. Considering the system as a whole, the analysis of complex thermal interaction of adjacent elements could be averted while the heat absorption and transmission of frame could be considered. The temperature of outer frame was set to be equal to the ambient temperature as a boundary condition. Although it was not so flexible compared with FEM analysis when dealing with the boundary condition, the theoretical model was proved to correspond with the experimental result, and could be used as an approximate formula in thermal response calculation of substrate-free FPA. The model avoids the complication of FEM analysis, especially for large arrays. Further more, the model can be used for substrate-free FPA dimensions design and optimization under certain response target.