The absorption characteristics of circular, offset, and rectangular double-clad fibers are investigated with simple and comprehensible 2D models. In the model, the pump modes in the first-cladding are divided into absorbable and unabsorbable modes. For circular fiber, a concept of absorbable power ratio is defined to characterize the effective absorption. For offset and rectangular fibers, almost all pump modes are absorbable, it is possible to achieve very high absorption efficiency. Such 2D models have been verified by 3D ray tracing simulations. The influences of offset distance for offset fibers, as well as cross sectional shapes for rectangular one on absorption are also discussed. The propagation losses of pump light in rectangular double-clad fibers are characterized both experimentally and theoretically. The measures to reduce pump loss in use of rectangular double-clad fibers are proposed. At last, the two ends of an Nd-doped rectangular double-clad fiber are bundled together to achieve efficient matching with a 10 W fiber coupled LD. With such a configuration, about 5.4 W pump light is launched into the first-cladding and 2.8 W laser output is obtained. A slope efficiency of 58 percent with respect to launched pump power has been achieved.