The 1.06 μm laser with irradiance 3.5×1013W/cm2 was injected into Mg microtube target with lateral jet nozzle. The temporal evolution of X-ray spectra emitted from Mg10+ and Mg11+ ion along the side-blow direction was recorded by using X-ray streak camera. The results demonstrate that three-body recombination is the main mechanism to realize the strong population inversion between the excited state energy levels Is4p and Is3p of Mg10+ ion. The electron density in lateral jet nozzle measured by a 2660? UV laser probe coincides with density results deduced from the spectrum information.