The invention provides a process and a system for mixed burning of coal slime in big proportion in a coal-powder boiler. The process comprises the following steps that coal slime is pre-treated, that is, the coal slime is subjected to impurity removal, crushing and paste manufacturing, so that the coal slime is suitable for pumping; the coal slime is pumped, that is, coal slime paste is hermetically transported in a coal slime powdering unit through a pipeline; the coal slime is powdered, that is, the air inlet temperature of a coal mill is increased and the air volume is increased by adopting a fuel gas heating or high-temperature steam heat exchanging method, and coal slime powder meeting the burning requirement is obtained; and the coal slime powder is fed in the coal-powder boiler in a gradient manner for burning, that is, the coal slime powder is fed in the coal-powder boiler in a gradient and layered manner for burning. The process and the system realize mixed burning of coal slime in big proportion, the operation stability and reliability of the system are improved, the fire coal resources in the boiler are saved, the effects of energy conservation and emission reduction are remarkable, and the pollution of wastes on the environment is eliminated.