This invention discloses a backplate comprising a sheet metal member with a hollow structure and an aluminum extrusion heatsink connected to the end of the sheet metal member. This aluminum extrusion heatsink includes a lateral light source connection portion provided with a light source and a sheet metal member connection portion connected to the lateral light source connection portion, wherein the latter is in contact with the sheet metal member. The aluminum extrusion heatsink further includes two heat-dissipation extension portions respectively arranged at both ends of the sheet metal member connection portion and extending in a length direction of the sheet metal member. Herein, the heat-dissipation extension portion at both ends of the aluminum extrusion heatsink can enhance the heat dissipation effect at the corner region. When the backplate is applied to a liquid crystal display device, it can reduce the stress concentration degree at the corner region thereof.