In the spaceborne gravitational wave interferometric detection, the problem of stray light has received long-term attention. The laser light emitted by the local interferometer produces backward coherent stray light when passing the telescope while the radiation from space that is incident to the spacecraft produces forward incoherent stray light. Forward incoherent stray light has received less attention at this point, but it is a necessary factor of gravitational-wave telescope design. Therefore, this paper studies stray light produced by space gravitational wave telescopes in orbit. First, the annual solar angle is calculated according to the orbital data of the three-star satellite formation of the Taiji Project, and the solar radiation around the 1064 nm band is evaluated. The baffle shadowing function is derived, which satisfies the requirement for the baffle design. The telescope is then modeled optically and mechanically and scatter measurements are conducted for critical optical components. Finally, the stray light reaching the pupil of the telescope is determined based on the energy of the incident sunlight. The results show that when the angle between the incident light and the optical axis is 60°, the stray radiation at the exit pupil is 3.9×10^-12 W, and the corresponding point source transmittance is 8.7×10^-9 which meets the requirement for space gravitational waves to detect extremely low levels of stray light.