The influence of Rashba effect on the properties of polaron in a triangular quantum well is investigated with LLP variational method and the expression of the polaron ground state energy is derived. The relations of the ground state energy with the wave vector, electron areal density and electron-phonon coupling strength are discussed, respectively. Results show that the ground state energy is an increasing function of the wave vector and electron areal density, and it is a decreasing function of the electron-phonon coupling strength. Due to the Rashba effect, the ground state energy splits into two branches.