The present invention relates to the technical field of porous composite material preparation, and provides a slurry synthesis method for a porous composite framework material. A mass ratio of a solvent to a solid reactant is controlled, so that the reactant is at different saturation levels, an inorganic-organic composite framework material is obtained by heating in a sealed container, and the grain size and distribution of the composite framework material are regulated and controlled. At room temperature, an inorganic metal source is dispersed into the solvent, and then mixed with a polycarboxylic acid organic compound to form slurry, the slurry is sealed in a self-pressurized reaction kettle, and heated to 70-170ºC, reaction is performed for 0.5-240 h, drying is performed at room temperature, and therefore the porous composite framework material is obtained. According to the method, the grain size is controllable, and the method provides a novel synthesis route for controlling the grin size of the composite framework material. The yield is increased, washing is not required, operating costs are low, the operation is easy, zero emission is implemented, the method is environmentally friendly and economically feasible, and it is easy to implement large-scale intensive production. The obtained large-grain composite framework material has a larger adsorption capacity in the aspect of gas adsorption.