The paper summarizes the nonlinearity and heat effect in double-clad fiber laserwith high power. When the output power of laser is low, the nonlinearity and heat effect have little influence on laser characters. But in Yb-doped double-clad fiber laserwith high power, especially kilowatt power , the nonlinearity and heat effect become the chief factor to limit the increase of laser power. By increasing the radius of double-clad fiber and shortening fiber length,the nonlinearity threshold will be enhanced, which can ensure that no nonlinearity will appear when optical signal power is increased. By using distributed pump, the temperature along fiber length will be the lowest and most uni-form, and the output power of laserwill be the most.