The invention discloses a manufacturing method for a rack of a marine low-speed diesel engine. The manufacturing method comprises the following steps that firstly, the rack is divided into a rack head segment module, a rack middle segment module and a rack tail segment module, wherein the rack middle segment module comprises a plurality of rack middle segments which are the same in structure and size; secondly, according to the design size of the rack, all part plates of all the rack segments are cut for blanking, then grooves are processed and welded, and finally the plates are stored in a classified mode; thirdly, a CO2 welding method is adopted for welding the rack head segment module, the rack middle segment module and the rack tail segment module step by step respectively; fourthly, the rack head segment module, the rack middle segment module and the rack tail segment module are jointed and folded; and fifthly, annealing treatment is finally conducted on connection weld joints to reduce residual stress produced by welding of connection areas, and after overall heat treatment on the rack is completed, aging treatment and flaw detection are conducted to make the rack reach all design standards. The manufacturing method has the advantages of being high in production efficiency and good in interchangeability and universality.