Owing to the high Ca content but low concentration of Si and Al in fly ash (FA) generated from municipal solid waste incineration (MSWI), hydrothermal treatment (HT) of MSWI FA directly could not result in the synthesis of calcium containing aluminosilicate minerals zeolites such as tobermorite, an important material for the hydrothermal solidification of heavy metals. According to the research results from the effect of the extra silicon-aluminum source added, with the higher amount (30%) of the composite additive in which the ratio of quality for coal fly ash (CFA) and diatomite is 1:1, tobermorite could be successfully synthesized, which is mainly attributed to the proper element proportion of Ca to Si and Al and Al to Si and Al, adjusted by additives. Furthermore, the tobermorite seed added also could not help the formation of tobermorite, even at a higher reaction temperature of 200 degrees C, due to the improper element proportion with a 10% mass of the composite additive. With a 30% mass of the composite additive and seed at a higher reaction temperature of 200 degrees C, the tobermorite seed added could not only promote the formation of tobermorite as early as the first 3 h but also inhibit the hibschite generated from tobermorite. Due to the leaching toxicity of Pb decreasing to the lowest level (0.25 mg/L) being related to the tobermorite largely formed, it can be concluded that the addition of a 30% mass of composite additive (including CFA and diatomite) and a 3% mass of tobermorite seed at a higher reaction temperature of 200 degrees C would be the recommended technological parameter for seed-assisted hydrothermal solidification of MSWI fly ash.