The energy level population rate equations are solved in collisional-radiational (CR) model, when the ground state of MgX, MgXII, MgXIII and the 21 energy levels of MgXI, of which the main quantum number n are lower than or equal to 6, are included, and the electron temperature drops vs time in Gaussian profile are assumed Then the variations of ASE gain coefficient of transition 1s3p-1s4d, 1s3d-1s4f in MgXI plasma with time are given.