Compared with fiber amplifier, fiber laser oscillator has the advantages of compact structure, good stability, high threshold of mode instability and good beam quality. Single-end pumped 1 kW all fiber laser oscillator is studied in detail theoretically and experimentally. Rate equation model which considers the fiber bending loss, mode coupling, changing pumping wave and cladding lights stripping is presented. Using a China made combiner, a single-end pumped all fiber laser oscillator is built. Laser power of 1.04 kW and optical to optical efficiency of 69% are obtained when the pump power reaches to 1.5 kW. The beam quality is measured and the results show that the M2 factors are all less than 1.25 in different powers. With one hour steady laser operation, the power fluctuation is less then 1%.