Mixed mortar is a new type of mortar that is prepared by using gangue machine-made sand and eolian sand to replace river sand in different proportion (equal mass). To explore the change rule and microscopic mechanism of the basic mechanical properties of mixed mortar, the mixed mortar specimens were prepared with the eolian sand of Mu Us Desert and the gangue machine-made sand of the Daliuta Coal Mine. The compressive strength change rules of mixed mortar with the two kinds of sand mixed ratio, curing age, and mixed sand fineness modulus were studied, and the internal mechanism of the strength change rule was revealed. The results show that with the increase of the proportion of eolian sand, the strength of mixed mortar increases first and then decreases, and the optimal mixing ratio of gangue machine-made sand and eolian sand is 6∶4 (mass ratio). The compressive strength of mixed mortar and the fineness modulus of mixed sand satisfy the quadratic function relationship. When the fineness modulus of mixed sand is 2.20, the mechanical properties of mortar are the best. The particles of different particle sizes of mixed sand fill each other, which reduces the internal pores of the mortar. The addition of an appropriate amount of eolian sand can disperse the large bubbles retained in the mortar into tiny bubbles and improve the structure of the mortar. The concepts of beneficial pore and harmful pore are proposed. Based on the pore structure parameters, the change process of mechanical strength of mortar is explained, which shows that the compressive strength of mortar is the result of the combined action of its internal porosity and pore size distribution range. The appropriate amount of aeolian sand mixed with gangue machine-made sand is conducive to the hydration reaction, which has a “compensation effect” on the structure. However, with the further increase of aeolian sand content, due to the defects of aeolian sand itself, this “compensation effect” cannot make up for the defects of the material itself.