The problems of direct pumping and two-stage pumping for high power fiber lasers are theoretically analyzed. The simulation results show that 1070 nm laser directly pumped by 975 nm laser provides a theoretical slope efficiency up to 80%. But it is hard to make the highest temperature of the fiber core reduce below 150 ℃ by forced water cooling when pump power is 10 kW. For the two-stage pumping technology, if the conventional cladding pumping technology is used the slope efficiency is less than 20% when a 1070 nm laser is pumped by a 1018 nm laser. With the pump power filling factor increasing from 0.0025 to 0.1, the slope efficiency can be raised from 18.5% to 80.9%, thus the total slope efficiency is raised from 15.5% to 68%. Two-stage pumping is inferior to direct pumping in terms of slope efficiency, but it has the obvious advantage of thermal management. How to raise the pump power filling factor to achieve high slope efficiency and good temperature characteristics at the same time is the key technology of the two-stage pumping.