Abstract: The external cavity laser (ECL), which is mainly used for coherent light communication, is characterized by narrow line width and low power consumption. In practical application, the output optical power of ECL will decrease when the ambient temperature is the high or low temperature. Through the use of finite element commercial software Ansys, it was found that the mechanical structure had the slight deflection change at high or low temperatures. Compared with the room temperature, the maximum deflection angle at high or low temperatures was 0.04°. The cavity length of ECL was long, combined with the optical path simulation software Zemax, the slight deflection change would cause the big change of optical path, affect the coupling efficiency and then lead to the reduction of output optical power. The change of optical path was confirmed by the experimental measurement, and based on the theory of power reduction caused by the deflection change, the methods of shortening the cavity length and thickening the base were proposed to reduce the variation of output optical power with the ambient temperature from the point of design. The proposed model and ideas can provide quantitative analysis ideas for the correlation analysis of power and temperature of similar products.