Based on the axial bed depth model of straight rotary kiln proposed by prior researchers，the equation of mean residence time of solids in carbon rotary kiln with variable diameter and turn-down rig was developed. Cold experiments were carried out on a laboratory rotary kiln under ambient conditions. The comparison between calculated results and experiment data showed that calculated results of the residence time of solids agreed well with measurements and the prediction error varied in a small range of -15%—+12%. No fitting parameter was necessary. The mass flow rate of solids increased and the mean residence time decreased with increasing kiln rotation speed and kiln slope. The mass flow rate of solids increased and the mean residence time might possibly decrease or increase with increasing dam height at the feed end.