The microtube target with side jet burner is designed. Strong population inversion is observed along the whole side jet burner between the energy leveles of MgXI Is3p 1P00 and Is4p 1P00 while the average laser irradiance reached about 3.5×1013W·cm-2 on the inner wall of the microtube targets. The corresponding environment average electron temperature 470-520 eV and density of 2×10cm-3 of plasma at the inversion area is measured. The possible mechanisms of population inversion under the average high temperature condition has been analysed.