The present disclosure provides a preparation method for hydrothermal synthesis of fly ash silicate aggregate including: mixing sodium metasilicate, potassium hydroxide, and inorganic-organic hybrid excitation monomer as raw materials to obtain an inorganic-organic composite activator; preparing a silicate aggregate raw material, mixing measured fly ash, carbide slag, quicklime, and vitrified micro bubble by mass, adding the inorganic-organic composite activator and continue stirring to produce a mixture; forming a ball disc, wetting an expanded perlite that forms a core of the ball by spraying water, adding a prepared mixture, spraying water while adding, standing and curing, performing a maturation and activation treatment in an autoclave, undergoing a silicon calcium reaction for a hydrothermal synthesis to obtain the silicate aggregates. The present disclosure obtains silicate aggregates with high-performance by accelerating an internal activity of fly ash at an early stage and fully activating the activity of fly ash in all process.