Tunnel cascaded and coupled multi-active regions laser diodes are novel high power laser diodes. This kind of laser diodes can achieve high output power at relatively low current density and overcome the main hindrance of the normal high power semiconductor lasers: catastrophic optical damage (COD) by increase the size of facula. Transient thermal property of these laser diodes has been calculated by using finite element method (FEM). Three kinds of laser diode structures, one active region, two active regions with one tunnel junction and three active regions with two tunnel junctions, are simulated. The calculated results are in agreement with the measured data. The result indicates that for tunnel cascaded and coupled multi-active regins laser diodes, temperature rising of the active region near the substrate is a little higher than that near heat sink. With active region number increasing, the temerature of the laser diodes rises but multi-active regions were fabricated on the uniform substrate, its thermal resistance is still smaller than that of series with the same number normal laser diodes.