Abstract: We demonstrated a novel fiber laser system with oscillating-amplifying integrated configuration and a 5 kW output power was achieved experimentally for the first time with optical-to-optical efficiency of 80%. The reliability and the boot order of the laser system were studied intensively at high power level. When the laser system is deliberately turn on in a wrong order, such as the pump of the amplifying section is switched on before the oscillating section, or the pump of the oscillating section is switched off before the amplifying section, which is normally a disaster in a classic main oscillator power amplifier (MOPA) system, the laser system can trouble-free operate at full power level. For the first time, it is verified that there is no priority between the amplifier and the seed in the oscillating-amplifying integrated fiber laser system.