The invention provides a method for constructing a gram-negative bacterium capable of expressing a gram-positive bacterium polysaccharide. The method comprises the following steps: transforming a low-copy expression plasmid of the gram-positive bacterium polysaccharide into an asd and rfbP gene-deletion gram-negative bacterium, screening by a balanced lethal system to obtain the gram-negative bacterium capable of producing the gram-positive bacterium polysaccharide. The invention also provides the gram-negative bacterium capable of producing the gram-positive bacterium polysaccharide prepared by the method. The invention also provides a mutant of Salmonella enterica serovar Typhimurium P0005, which is named as Salmonella enterica subsp. enterica serovar Typhimurium P0005, and has the CCTCC NO: M 2016344. The gram-negative bacterium can express the gram-positive bacterium polysaccharide, and the method provided by the invention does not contain any resistance marker and meets the biosafety requirements of subsequent vaccines and other application.