The invention discloses a downward slicing advancing filling body strength testing method comprising the steps: performing sampling design and sampling water samples in a the side of a filling body exposed from a two-step stoping jamb; according to the strength of a circular cylinder test block obtained by field sampling, establishing the relationship between the circular cylinder test block and a cube test block by a laboratory experiment to obtain the strength of a slicing filling body cube; comparing the strength of the advancing filling body cube and the design strength of the filling body to obtain a strength safety index of the field filling body; and finally, according to the safety indexes of the strength of the filling body at multiple sampling positions on two sides of the filling body, drawing a safety index isoline of the whole filling body advancing, evaluating the stability of the advancing filling body and predicting risks of catastrophe and unstability of the filling body. The method can be used for providing a strength testing and safety evaluation technology for safe mining under the top of the filling body so as to improve safety production and risk prediction ability during mining by using a downward advancing filling body method.