The invention discloses an automatic mixing device for a mixed feed additive. The automatic mixing device comprises four parts, namely a premixing system, a mixing system, a temperature control system and an automatic control system, wherein the premixing system comprises a premixing bin body, a main and auxiliary material feeding hole, a mixing blade, an adjustable motor and a fixed sealing flange; the mixing system comprises a mixing bin body, a mixing blade, an adjustable motor and a vibrator; the temperature control system comprises an internally inlet / outlet, a seal sheet and an interlayer; and the automatic control system comprises a relay switch gear, a connecting circuit and an automatic control board. The automatic mixing device has the characteristics of high automation, convenience in use, high main / auxiliary material mixing strength and obviously improved mixing degree, is high in device sealing degree, is capable of lowering the pollution rate and the material loss rate, reducing damage to human bodies, decreasing heat produced by machine operation and improving the activity and heat-labile component quality of the mixed feed additive, and is beneficial to quality stability of products in different batches.