The invention discloses a preparation method for mullite in-situ reinforced silicon carbide porous ceramic sintered at low temperature. The preparation method comprises the following steps: subjecting Al2(SO4)3, Na2SO4 and B2O3 to mixing and ball milling so as to obtain a mixture A; then subjecting the mixture A, SiC powder and a pore forming agent to mixing and ball milling so as to obtain a mixture B; subjecting the mixture B to compression molding and then carrying out drying and calcining; and successively carrying out ultrasonic cleaning and drying on the calcined sample. The method promotes low-temperature sintering of silicon carbide porous ceramic; the method overcomes defects in usage of the silicon carbide porous ceramic at high temperature in the presence of a high-temperature binder or sintering agent and is favorable for further improving the porosity of the silicon carbide porous ceramic; and more importantly, SiO2 produced on the surface of silicon carbide due to oxidation is converted into a mullite crystal phase through a high temperature reaction, so the high-temperature mechanical properties and antioxidation performance of the silicon carbide porous ceramic are further improved.