The invention discloses a system and a method for improving slurry property of low order coal slurry. The system includes a powder making system and a fine powder separating system. The powder making system comprises two vertical mill mechanisms. The vertical mill mechanisms are connected with bucket elevators which are respectively connected with a coarse powder finished product cabin and a fine powder finished product cabin. The fine powder separating system comprises a buffering cabin connected with the fine powder finished product cabin. The buffering cabin is communicated with a first grader and a second grader in sequence. The first grader and the second grader are respectively communicated with a coarse powder collecting cabin and a super fine powder collecting cabin. The coarse powder finished product cabin is communicated with a slurry mixing system through a dense phase conveying device; and the fine buffering cabin is communicated with the slurry mixing system through a conveying belt. The method includes three parts of coarse powder preparation, fine powder separation and extraction, and high-density coal slurry preparation. By collocating proportion of coarse coal powder and fine coal powder, deposition efficiency of coal powder is improved, distribution of coal powder after being mixed is close to an Alfred model; and slurry property of low order coal slurry is improved through graduation.