Abstract: The backfill mining method is widely used in mines because of its advantages of environmental protection, safety, and efficiency and includes filling slurry pipeline transportation as an important part. The rheological parameters of filling slurry are important indicators for evaluating the characteristics of filling slurry pipeline transport; these parameters are mainly determined by rheometers at present, while the rheological experimental conditions are usually unavailable at the mine site. Because of its simplicity and speed, mines mainly use a slump test to evaluate the flow properties of a filling slurry. In this paper, we used a mini-slump cone to conduct a slump experiment of filling slurry with different mass fractions and cement–tailings ratios (the two most common variable parameters in filling slurry ratios), established an analytical model between the spread of a mini-slump cone and yield stress, obtained a simplified calculation model according to the shape of the filling slurry after flowing, calculated the yield stress of the slurry based on the simplified model, and compared the theoretical value with the experimental value of the yield stress of the filling slurry under the condition of the same ratio tested by a rheometer. At the same time, the influence law of different mass fractions and cement–sand ratios on the expansion degree of the filling slurry was studied using a two-factor analysis of variance. The results show that the spread is mainly influenced by the mass fraction and unsubstantially affected by the cement–sand ratio. The yield stress of the filling slurry increases with the mass fraction. When the mass fraction is low, the error in the theoretical value relative to the experimental value has a large range, and the error in the theoretical value is within 25%, averaging 16.79%; as the mass fraction increases, this error gradually decreases below 15%, averaging 8.81%. Considering its effect, a correction factor based on the mass fraction was proposed, and the error in the theoretical yield stress value after the correction was reduced below 10%, averaging 3.54%. In this study, the mini-slump cone test not only reduces the experimental material and labor intensity compared with the traditional slump test but also effectively characterizes the yield stress of the slurry, which provides practical guidance for evaluating the flowability of mine filling slurry.