Abstract: A novel laser model with active gain medium and open-resonator boundary condition was developed, which is applicable to both stable and unstable resonators. Based on the equivalency between the Schrdinger equation and Helmholtz equation, quantum mechanics method was used to solve the Helmholtz equation of resonators, and the eigen equation and eigen solution were obtained, which fully satisfy the wave equations, boundary condtions, matrix optics, and stability criterion. Based on the eigen equation, the relationship between transverse mode number and output coupling ratio was derived. Futhermore, the relationship of the output power and output coupling ratio to magnification was deduced. This model not only can regress into the exact form of the traditional formula in case of small output couplings, but also consists with the curve-fittings of experimental data very well in case of large output couplings.