Porous aggregates are a key material for the lightweight refractory. Porous magnesia based composite spinel (Mg (Fe, Al)₂O₄) refractory aggregates were prepared by an in-situ decomposition synthesis method with calcined magnesite, Al(OH)₃ and iron oxide as raw materials. The influence of sintering temperature (i.e., 1 000-1 600 ℃) on the microstructures and strengths was investigated, and the synthesis mechanism of porous aggregates was analyzed. The results show that the pores in the samples are composed of the pores in the micron-sized particles and the pores among micron-sized particles as well as they have a few neck connections at the sintering temperatures of 1 000-1 300 ℃, and the pores in the micron-sized particles migrate to the gaps and the necks grow with a further increase of sintering temperature to 1 400-1 600 ℃, thus promoting the compressive strength of the samples. The formation of composite spinel (Mg (Fe, Al)₂O₄) in the samples involves that MgAl₂O₄ is formed on the surface of Al(OH)₃ pseudomorph and reacts with Fe³⁺ to form the composite spinel (Mg (Fe, Al)₂O₄) with a ring structure, and MgFe₂O₄ is formed in the position of MgO and reacts with Al³⁺ to form the composite spinel (Mg (Fe, Al)₂O₄) with the granular and strip structures. The porous magnesia based composite spinel (Mg (Fe, Al)₂O₄) refractory aggregates have the optimum performance (i.e., an apparent porosity of 24.4%, a median pore size of 25.89 μm, a thermal conductivity of 2.79 W/(m·K) at 1 000 ℃ and a compressive strength of 74.4 MPa) at the sintering temperature of 1 550 ℃. Meanwhile, compared with the existing sintered magnesia, the thermal conductivity of porous magnesia based composite spinel (Mg (Fe, Al)₂O₄) refractory aggregates synthesized can be reduced by 64.7%.