Beta induced Alfvén eigenmode (BAE) can be an important candidate for ion loss in burning plasmas. Effects of tokamak elongation on BAE has been investigated by the gyrokinetic eigenvalue code DAEPS in this work. We construct a shaped tokamak equilibrium model by modifying local s-α model to study the elongation effects. It is found that the BAE growth rate first increases with larger elongation factor κ, then reaches a maximum, and finally decreases. This trend occurs for a large range of ion temperatures. In the weak or moderate elongation region, the BAE instability is a reactive type and mainly determined by the fluid/MHD effects, namely the combination of stabilizing field line bending term and destabilizing interchange drive in the vorticity equation. However, in the strong elongation region, the BAE instability becomes dissipative and is mainly driven by the wave-particle resonances. In this region, the wave-particle resonance finally decreases with a very large elongation, which is due to the decrease of the geodesic curvature with elongation.