Fiberglass-reinforced plastic (FRP) composite sheet pile has relative low elastic modulus and the shape of the thin-walled structure in which excessive deflection will occour. And this will effect its normal operation. The deflection of FRP sheet pile includes bending deflection and shear deflection. A simplified method is introduced to calculate the deflection of FRP sheet pile wall. The maximum deflection obtained from this method is very close to that from finite element method; and it is shown that the method proposed is quite reliable.