A mathematical model of the mirror misalignment of a four-equal-sided nonplanar ring cavity is established in this letter. The variations in the optical axis are discussed through an augmented 6 × 6 ray matrix formulation. Numerical analysis shows that the self-consistence of the optical axis can always be obtained because of the thermal effects in any case of mirror misalignment. For a fabricated design, optical axis variations always appear. The influence of thermal effects (i.e., pump power) on optical axis variations are studied. The tilt of the optical axis remains constant, whereas its decentration varies with pump power. Further analysis shows that the actual closing point of the optical axis moves close to the ideal point as the pump power increases. The theoretical analysis proposed is proven and validated by the experimental results.