Experiments show that transmittance of parallel-plane Ge changes quasi-periodically with time under CW CO2 laser radiation. Basing on multi-beam interference theory for parallel-plane and considering influence of laser-induced temperature change on refractivity and thick of Ge, calculated result is consistent with experimental data. It indicated that parallel-plane Ge has a dynamic Fabry-Perot interference effect as dependences of refractivity and thick on temperature by high power CW CO2 laser heating.