For better understanding the light propagation in a double clad fiber (DCF), the perfect circular double clad fiber is numerically investigated by using the coupled mode method with translating the DCF into a single mode fiber (SMF) and an annular core fiber (ACF).The coupling coefficients between the LP01 mode of the SMF and the guided modes of the ACF are calculated. The results show that the LP0n modes of the ACF have greater coupling coefficients than the other guided modes, and for the LP0n modes the coupling coefficient of the high-order mode is larger than that of the low-order mode. The field distribution in the DCF core calculated by the coupled mode equations shows a quasi-periodic distribution along the DCF. The average period of the field distribution increases with the increasing of wavelength for different DCF parameters and the average normalized power of the field distribution is closely related to the chosen parameters of the DCF.