We established a formal series symmetry theory for a type of generalized 2+1 dimensional bilinear equation in two different ways. Starting from a known time in-dependent symmetry or an arbitrary function of 1-D space, we can get a formal series symmetry with an arbitrary function of time t. For the 2+1 dimensional bilinear Sawada-Kotera equation, there exist six truncated symmetries. These truncated symmetries constitute an infinite dimensional Lie algebra. Some significant subalge-bras such as the Virasoro algebras are also given.