The invention relates to a filling mining method for a slant middle-thick ore body. The method includes the following steps: (1) chamber division; (2) chamber stoping; specifically, each chamber is divided into two mining ranges respectively of a hanging side and a heading side, namely, a hanging side mining range stope and a heading side mining range stope, according to the connecting line between the hanging side vertex and the heading side bottom point for stoping; the heading side mining range stope is preferentially stoped, and then the hanging side mining range stope is stoped; in each stage, the stoping is carried out in the chambers one by one from bottom to top along the ore body; the stoping in both the heading side mining range stope and the hanging side mining range stope includes stope arrangement and three processes, namely, mining, stope ore drawing and chamber filling; the chambers are divided into two mining ranges, the hanging side mining range and the heading side mining range, for mining; the heading side mining range is firstly mined, and after the goaf is filled, the hanging side mining range is mined, so that full recovery of collapsed ores in the heading side mining range stope can be ensured, and untimely falling of the surrounding rock on the hanging side due to crush and mixing-in of a large number of barren rocks on the hanging side are avoided; the hanging side mining range stope is stoped after the goaf of the heading side mining range stope is filled in a high-concentration manner, so that stability of the stope surrounding rock body is improved; by partially controlling the ore drawing, ore piles are reserved in the stopes, so that the hanging side surrounding rock can be effectively supported, an extrusion condition is provided for next blasting, and the blasting effect is improved.