Abstract: Thermal effect of high power fiber laser severely limits the output power and beam quality.Temperature distribution in a double-clad high power fiber laser was analyzed based on thermal conductive equation and boundary condition,and thermal stress distribution was presented by temperature distribution,the change of index refraction induced by temperature and stress was analyzed separately,optic path difference in double-clad high power fiber laser induced by thermal effect was presented.Results indicate that all stresses are compressive,in the fiber core region,however,in the cladding region,the stress is partly compressive and tensile;at the fiber outer boundary,the radial stress is zero while the tangential and longitudinal stress are equal and tensile;the change of index refraction induced by temperature is larger than by stress;optic path difference was caused mostly by the change of temperature,very small by thermal expand and thermal stress.