The working mechanism of dual-wavelength laser diode cascaded by tunnel junction was introduced and heat source of the device was analyzed.By using the finite element method to solve the thermal conduction equation,the thermal distribution of the laser diode at pulse current was obtained.It is found with time lapsing the temperature of the active region close to the substrate is higher than that of the active region close to the heat sink.The temperature variety trends at differnet duty ratio were calculated.With the duty ratio increasing,heat in the active regions accumulate gradually,it would influence the device characteristic.