Based on the nodal aberration theory, a special case of misalignments for reflective optical systems, which satisfying the condition of the coma-free pivot point, was studied. Under the effect of the special misalignments, the full field display aberration characteristics of the 3rd order coma and the 3rd order astigmatism were analyzed, and the analytical calculation formula of the two nodes position of the 3rd order astigmatism was established. It was found that the 3rd order coma would not change, and one node of the 3rd order astigmatism would locate near the central FOV (field of view), which revealed the drawback of using only coma-zero in axial FOV as the criterion of perfect alignment during the traditional optical assembly. Based on the analysis of wavefront aberration, a method to judge whether the system was aligned well according to the variance of wavefront aberration in only one axial FOV after introducing quantitative misalignments was proposed. A type of two-mirror optical system was adopted by CODE V(Version 10.2) to simulate the effect of misalignments on wavefront aberration, results show that the model and the method established can quantitatively analyze the two nodes position of the 3rd order astigmatism and verified the validity of the proposed judgement method for system assembly.