The invention discloses a preparation method of high-flame-resistance and heat-heat-conductivity composite particles based on magnesium hydroxide. The composite material does not contain any halogen or non-environmentally-friendly metal flame retardant and the flame-retardant grade reaches to V-0 level; meanwhile, high heat conductivity is achieved and the heat-conducting coefficient is more than0.5 W/m.DEG C. Flaky magnesium hydroxide slurry and graphene microchips are premixed through grinding means to form magnesium hydroxide and graphene microchip mutual embedded mixed slurry, and the mutual embedded type structure is converted into a spheroidal composite spherical powder by a spray-drying method and by utilizing the action of temperature and a gravity force. The preparation process is stable, high in finished product rate, wide in application range and suitable for batch production. The high-flame-resistance and heat-heat-conductivity composite particles based on the magnesium hydroxide can be directly applied to radiators such as LED, electric appliances and electronic components, so that the heat can be dissipated into a cold environment and the operation temperature of electronic devices can be reduced.