The long-term, efficient and low-cost immobilization of the heavy metals contained in municipal solid waste incineration (MSWI) fly ash, is an important path to realize its harmless and resource treatment. The disposal of MSWI fly ash into geopolymers through alkali activation technology (AAT) can effectively simultaneously immobilize heavy metals and reuse MSWI fly ash. However, how to reduce the dosage of alkaline activator (AA) and improve the compressive strength of treated materials has always been a difficulty in the field. In this study, metakaolin (MK) was added (0 −20 wt%) as an aluminosilicate source to explore its effects on the compressive strength and heavy metal immobilization efficiency of the geopolymers; additionally, mold-pressing method was used to investigate its feasibility of reducing the consumption of AA. The results revealed that, in the case of the 20 wt% MK samples, the compressive strength of the geopolymers can significantly increase to 30.31 MPa, meanwhile, the AA requirement is only 25 wt% of the MSWI fly ash and MK, and the leaching toxicities of various heavy metals were below the limit established by the Chinese standard (GB 16889–2008). The content of free CaO in the optimal samples was observed to decrease to < 1 wt%. In addition, the results revealed that an increase in mold pressure tended to coincide with an increase in the proportion of gel pores (up to 6.98%), which increased geopolymer compressive strength. The main products yielded by the mold-pressed MSWI fly ash geopolymer are sodium-aluminosilicate-hydrate (N-A-S-H) gels.