The invention discloses a rotary kiln molybdenum oxide roasting system for supplying oxygen to material bottom by utilizing residual heat. In the rotary kiln molybdenum oxide roasting system, one end of a hot air channel in the rotary kiln molybdenum oxide roasting system is connected with a residual heat utilizing system, and the other end of the hot air channel is used for introducing hot air in the hot air channel to a position right below materials in a rotary kiln through a material bottom oxygen supply device. According to the rotary kiln molybdenum oxide roasting system disclosed by the invention, the material bottom oxygen supply device is adopted, so that the hot air after heat exchange is directly introduced to the bottom of the materials; the materials cannot be thrown down during rotation, circulating hot air is continuously introduced, so that the sufficient mixing of oxygen and the materials is guaranteed, the roasting effect is perfect, the effect for molybdenum trioxide is good, operation can be completed by roasting once, and inconvenience brought by repeated roasting can be avoided; a structure is changed from the aspect of roasting, so that the effect of roasting molybdenum dioxide is good; and through the adoption of the scheme of the invention, the productivity ratio of the rotary kiln can be increased for either molybdenum trioxide or molybdenum dioxide, and energy consumption can be reduced.