Abstract: The bremsstrahlung is an important source term in the shielding design of the electron accelerator. In order to study the characteristics of the photon source term in the 90° direction and the influence of target radius and thickness on the bremsstrahlung in the 90° direction, the Monte Carlo program of MCNPX2.7 was used to calculate the radiation source term from the 15 MeV3 GeV electron beam incident on the iron target with different thicknesses and radii. The variation characteristics of radiation source items such as photon radiation dose rate and photon energy spectrum with target thickness and radius were obtained and comparatively analyzed. By comparing with the photon source term in the 0° direction and the energy deposition of cascade electron in the target, the main features of the photon source term in the 90° direction were obtained. Finally, the effects of beam power and beam intensity on radiation dose rates in the 90° and 0° directions were analyzed. The result shows that, due to the combined influence of electron energy deposition in the target and self-absorption of photons from the target, the radius and thickness of the target are expected as important factors which affect the photon source term in the 90° direction. The energy of the cascade electrons deposited in the target and the radiation dose rate in the 90° direction increases with the thickness. As the energy of the cascade electrons is completely deposited in the target, the radiation dose rate in the 90° direction will not change with the target thickness. The radiation dose rate in the 90° direction also increases with the target radius, but when the target radius is larger, the radiation dose rate in the 90° direction decreases due to the selfabsorption. Apart from that, the effect of incident electron energy on the photon source terms in the 90° direction and 0° direction is different. More than 99% of the photons in the 90° direction are below 10 MeV, and most of them are within 3 MeV. In the 90° direction, the photon energy spectrum shape has little relationship with the incident electron energy. However, the photon energy in the 0° direction can reach the incident electron energy, which is generally much larger than the photon energy in the 90° direction, and the shape of the energy spectrum is greatly affected by the incident electron. As a suggestion, the influence of the target size should be considered in the shielding design of the electron accelerator, and the design in the lateral shielding should be optimized according to the difference between the photon source term in the 90° direction and 0° direction.