A method for preparing a large-size sintered samarium cobalt permanent magnet includes the following steps that firstly, a samarium cobalt alloy ingot is smashed, milled and mixed to obtain magnetic powder; secondly, oriented-pressing and forming are conducted in a magnetic field to obtain blanks; thirdly, two or more blanks are spliced together and tightly wrapped by fresh-keeping films, vacuum packaging is conducted, cold isostatic pressing is conducted, and a formed blank is obtained; fourthly, the formed blank is subjected to vacuum pre-sintering, inert gas is introduced for sintering, solid solution is conducted, air cooling is conducted to normal temperature, and a sintered blank is obtained; fifthly, the sintered blank is subjected to aging treatment, cooling and heat preservation are conducted, air cooling is conducted to normal temperature, and the large-size sintered samarium cobalt permanent magnet is obtained through machining. The ultra-large sintered samarium cobalt permanent magnet with the unilateral size larger than 130 mm can be prepared through the method, is good in magnetic property, and can reach the XGS30H trademark magnet standard, the product yield reaches 94% or above, the magnetic property is not changed after temperature-resistant and fatigue testing, and the appearance is normal. The process is simple, large-scale production can be achieved, and the cost is low.