This paper investigates the damaged area of a reinforced concrete beam under rectangular explosive contact explosion, through full-scale beam tests and numerical simulation. The calculation equation of beam surface load distribution based on equivalent impulse is established, with a consideration of the effect of the length and height of rectangular explosive on the load distribution, and the calculation equation of beam damage area is further proposed. Through changing the mass of the rectangular TNT explosive (1~6 kg) and the shape of the 1 kg rectangular explosive, 5 cases of the test were carried out on a full-scale reinforced concrete beam. The damaged area of the beam is divided into three parts: blasting crater, damage span of the front face, and damage span of the bottom face. The RHT (Riedel–Hiermaier–Thoma) material model is used to simulate concrete for numerical simulation. Curve fitting was performed based on the numerical simulation results. With the prediction of the load distribution on the beam surface, the size of the surface crushing area and the span of the damaged area are calculated; the section resistance function of the beam is introduced to calculate the depth of the blasting crater; and the correlation curve between the damaged span of the front face, the depth of the blasting crater, and the mass of the block TNT is established. The local damage to the beam under the contact explosion load can be evaluated more accurately when the mass of the rectangular TNT is 1~6 kg.