This paper focuses on the balancedness of symmetric Boolean functions. We prove a conjecture presented by Canteaut and Videau, which states that the balanced symmetric Boolean functions of fixed algebraic degree are trivially balanced when the number of variables is large enough. Denoted by σn,d, the n-variable elementary symmetric Boolean function of degree d. As an application of this result to elementary symmetric Boolean functions, we show that all the trivially balanced elementary symmetric Boolean functions are of the form σ2t+1l-1,2t, where t and l are any positive integers. It implies that Cusick et al.'s conjecture, which claims that σ2t+1l-1,2tis the only nonlinear balanced elementary symmetric Boolean functions, is equivalent to the conjecture that all the balanced elementary symmetric Boolean functions are trivially balanced.