In this paper, we investigate the spectrum properties of the lasing without population inversion in a three-level atomic system under an action of an externally applied microwave field. The probed absorption spectrum is obtained by solving the density matrix equation of this system. The results show that the absorption profile has a multi-peak structure, and exhibits obviously negative absorption (light amplification) in a specific range under the action of strong microwave field. The characteristic of the light amplification without population inversion according to the three level atomic system is demonstrated by the quantitative analysis of populations related to the probe levels.