The thermal effect and its impact on mode property of double-cladding photonic crystal fiber (DC-PCF) have been analyzed in this paper.The equations of heat conduction for the DC-PCF cross-section is given and applied to study t he thermal effect.The relationship between the radial temperature distribution o f the DC-PCF and its structural parameters,heat transfer conditions and thermal load are respectively studied.The impact of the thermal effect on the effectiv e refractive index of fundamental mode,effective mode field area and the mode f ield distribution are further given.It′s found that the radial temperature dis tribution is closely related to the structural parameters of DC-PCF.The highest temperature occurs at the fiber core,which increases as the air hole ratio inc reases.Besides,the temperature of the core decreases as both the heat transfer coefficient and thermal load decrease.The temperature gradient increases linea rly as the heat transfer coefficient and thermal load increase.The thermal effe ct also has impact on the mode field distribution.As the thermal load increase from 10W/m to 30W/m,the effective refractive index of the fundamental mode inc reases,the effective area of the fundamental mode linearly decreases,and the sh orter the wavelength,the more obvious the fundamental mode field shrinks,and t he more significant the thermal effect is.