A structure for reinforcing a seat mounting portion of a vehicle body may include a floor panel having a tunnel portion that protrudes in a height direction of a vehicle, side sill members extended in a length direction of the vehicle and coupled to the floor panel at both left and right sides in a width direction of the vehicle, a seat inner member coupled to the tunnel portion, a seat cross member extended in the width direction of the vehicle and having an outer tip portion coupled to the side sill members and an inner tip portion coupled to the seat inner member, a seat outer member coupled to the outer tip portion of the seat cross member and attached to the side sill members and the floor panel, and a reinforcing member inserted into the seat outer member to reinforce rigidity.