A novel fiber laser called an oscillating-amplifying integrated fiber laser was studied experimentally, in which the oscillating section and amplifying section share the pump between them. Based on this configuration, a 5-kW fiber laser system with optical-optical efficiency of 80.9% and M2 factor of 1.5 was achieved. The startup and shutdown sequence of the laser was studied in detail. When pumps of the laser were deliberately turned on in an inverted order, such as switching on/off the amplifying section before/after the oscillating section, which is normally disastrous in a classic fiber amplifier, the laser system turned out to operate stably at full power level. Thus, it is verified that there is no priority between the amplifier and the seed in this laser system. It combines the advantages of conventional fiber oscillators and fiber amplifiers, including high efficiency, high reliability, good anti-backreflection, and simple control logic.