## 1 Introduction  
In 2016, LIGO announced its detection of gravitational wave events [1], which is the first time that humans achieved direct detection of gravitational waves, thus, [PERSON]'s prediction about gravitational waves was confirmed. The ground-based gravitational wave detectors represented by LIGO are affected by ground vibrations, gravitational gradient noise, and short interferometric arm lengths, resulting in their low detection sensitivity, which can only detect gravitational waves at frequencies above 10 Hz [2]. However, for most of the gravitational wave sources in the universe, the frequency range of gravitational waves is between 0.1 mHz and 1 Hz [3, 4]. To achieve the detection of low-frequency gravitational wave signals, they must be free from the limitations of the ground environment to develop space gravitational wave detection. With high detection sensitivity, space gravitational wave detection can achieve the detection of more gravitational wave sources, including early cosmic phase transitions, cosmic strings, and dense binary systems. The detection of these sources can provide key information for the research of the early universe, unified field theory, and the evolution of the universe [5]. The Taiji Project is a space gravitational wave detection project proposed by the Chinese Academy of Sciences, aiming to detect picometer-level space gravitational wave signals within the frequency range of 0.1 mHz to 1 Hz [6; 7; 8]. Taiji plans to launch three satellites to form three laser interference links, each with a length of 3 million kilometers [9; 10]. In order to establish an effective laser transmission path between the two satellites, laser transceiver telescopes are designed at both ends of each interferometric link. The laser transceiver has two functions: one is to expand the outgoing beam from the scientific interferometer with a diameter of 2 mm into a collimated beam with a diameter of 200 mm, which is transmitted to the remote detector; the other is to narrow the signal beam and transmit it to the interferometer optical platform [11]. When the telescope emits the local laser, the laser power at the entrance pupil is close to 2 W, while the power of the signal beam received by the telescope from the remote detector is about 300 pW [9]. Backscattering is inevitable when a high-power outgoing beam passes through the telescope, and the backscattered light would interfere with the local reference beam, thus any phase fluctuation of the backscattered light would affect the measurement of the interference signal [12; 13]. Therefore, it is necessary to analyze and suppress the backscattered light of the telescope. In order to reduce the level of backscattered light, the optical system of the Taiji Telescope adopts an unobstructed off-axis four-mirror optical system. Due to the added tilt of the secondary mirror, the narcissus reflection is completely avoided [14; 15]. However, non-zero surface roughness is unavoidable during the manufacturing process of optical surfaces, and there should also be the possibility of contamination of optical surfaces, both of which would affect backscattering [16; 17]. In addition, the telescope will face the risk of micrometeoroid impacts once in orbit [18; 19; 20]. The high-speed impact of micrometeoroids will cause damages to optical surfaces, leading to increased levels of stray light. In this paper, the numerical analysis of the backscattering of the Taiji telescope has been carried out, and the influence mechanism of the backscattered light on the phase has been clarified through theoretical derivation with the requirements for the stray light of the telescope obtained. Subsequently, the precise scattering path of the stray light has been determined by the analysis of stray light on the Taiji telescope. The effect of optical surface roughness and cleanliness on the level of stray light has been studied.