A strengthened support structure for a net post assembly used in ball games comprises a plurality of lower support units disposed between two lateral support rod assemblies thereof, and each of the lower support units includes a first support section, a second support section, and a support body. The first support section and the second support section are connected to define an angle therebetween, and the support body is connected to the free ends of the first support section and the second support section. Each of the lower support units acts as an independent stress-sharing structure, so that stress can be equally distributed over the lower support units. The angle defined between the first support section and the second support section cooperate with the support body to strengthen the structural strength of the respective lower support units, which maintains the distance between the two lateral support rod assemblies constant.