The invention provides a variable frequency low speed mixer and a method for mixing materials with different stacking densities. The mixer comprises a main mixing chamber and an auxiliary mixing chamber, wherein the auxiliary mixing chamber is arranged in the main mixing chamber; a material mixing cover of the main mixing chamber is provided with a granular material feed port; the bottom of the main mixing chamber is provided with an impeller mechanism; the top of the auxiliary mixing chamber is provided with a powdery material feed port and a liquid material feed port; the wall of the auxiliary mixing chamber is provided with a liquid material discharge port and a plurality of powdery material discharge ports; the top of the auxiliary mixing chamber is provided with a liquid material temporary storage chamber which is communicated with the liquid material feed port and the liquid material discharge port; and by the method, the granular material or preliminarily mixed material is threw up by using the main mixing chamber and the powdery material or the liquid material is scattered in the main mixing chamber by using the auxiliary mixing chamber, so various materials with large stacking density difference are mixed uniformly. The mixer and the method can effectively prevent the phenomenon of non-uniform mixing of the materials, realize premixing with good effect and reduce the probability of demixing of the granular material and the powdery material while greatly shortening the mixing time.