In order to more conveniently and clearly analyze the optical model property of the resonator, the logic relationship between the propagation circle and transformation circle of the dual-wavelength laser resonator and the focal length and dynamic stability of the thermal lens of the gain medium is established with propagation circle analysis method. A quasithree-level thermal stable cavity containing a single thermal disturbance center is established. According σ circle and π circle tangent requirements, the relationship between the fundamental mode spot size and the curvature radius of M1 mirror in the gain medium of quasi-three-level laser is analyzed. By analyzing the thermal stability cavity of the quasi-three-level laser cavity with single thermal disturbance center, the position and ft2 value of the plane mirror M2 (the focal length value of the Ft2 lens) are determined.