To study the influence of mortar volume fraction on the compressive strength and working performance of metakaolin geopolymer mortar, the workability, rheological properties, and compressive strength of geopolymer mortar were tested with different mortar volume fractions as variables. The results show the hopping fluidity and consistency of slag-metakaolin geopolymer mortar decrease with the increase in mortar volume fraction, and the yield stress and plastic viscosity increase as the mortar volume fraction rises. The metakaolin geopolymer mortar exhibits excellent working performance when the mortar volume fraction is between 35% and 45%. The compressive strength of the slag-metakaolin geopolymer mortar increases first and then decreases with the increase in mortar volume fraction and reaches the maximum when the mortar volume fraction is 45%. Therefore, it is suggested that the mortar volume fraction should be 45% when slag-metakaolin geopolymer mortar is prepared, so as to ensure both good working performance and high compressive strength.