One method of non-uniform finger spacing is proposed to enhance thermal stability of multiple finger power heterojunction bipolar transistor. Using coupling thermal resistance to characterize the influence of the change of emitter spacing on thermal coupling, the relation between coupling thermal resistance and emitter spacing is obtained. Temperature distribution on the emitter fingers of multi-finger heterojunction bipolar transistor is studied by a numerical electro-thermal model. The results show that the heterojunction bipolar transistor with non-uniform finger spacing has a smaller temperature difference between fingers thant the traditional uniform finger spacing heterojunction bipolar transistor under the same power dissipation. So the method of non-uniform finger spacing is very effective to enhance the thermal stability.