## 1 Introduction  
In September 2015, the United States LIGO detector [1] for the first time directly detected gravitational wave signals and confirmed [PERSON]'s prediction about the existence of gravitational waves 100 years ago. The detection of ground gravitational waves is affected by the gravity and the size of the earth, and only mid-high-frequency signals above 10 Hz can be detected. Due to the existence of low-frequency gravitational wave radiation sources in space [2], space gravitational wave detection has become the next scientific hotspot for gravitational wave detection. In order to contribute to space gravitational wave detection, the Chinese Academy of Sciences started a space gravitational wave detection mission--Taiji Project [3; 4; 5; 6; 7]--in 2016. The plan will be implemented in three stages. The 1 st phase was to launch a technology verification satellite, which was successfully launched on 31 August 2019 [8]. One of the main tasks of the satellite is to verify the inertial sensing technology. Eventually, three space gravitational-wave detectors are expected to be launched around 2030 to detect gravitational waves in the frequency range of 0.1 mHz to 1 Hz. A high-precision space inertial sensor (IS, also known as gravitational reference sensor) [9] is an important device and key measurement equipment for the Taiji technology verification satellite. Based on capacitive displacement sensing and electrostatic force feedback control technology, the IS is highly sensitive to slow and weak external acceleration such as the radiation pressure from the Sun or Earth, or atmospheric drag [10], and can achieve multiple degrees of freedom and high-resolution results [11]. Therefore, it is widely applied to gravitational wave detection [12], the Earth's gravity field measurement [13], equivalent principle verification [14], spacecraft drag-free [15], and other space science applications. European and American countries have successfully launched scientific satellites such as CHAMP, GRACE, GP-B, GOCE, GRACE Follow-on, MICROSCOPE, and LISA Pathfinder [16; 17; 18; 19; 20; 21; 22]. The measurement resolution of these inertial sensors reached \(10^{-9}\)\(\sim\)\(10^{-14}\) m/s\({}^{2}\)/Hz\({}^{1/2}\). In order to overcome the effects of gravity and seismic noise on the ground, some special methods such as high-voltage levitation [23], suspension wire torsion pendulum measurement [24], and drop tower test [25] are employed to evaluate and test the performance of IS. Based on the LISA Project, the University of Trento in Italy [26] and the University of Florida [27] in the United States designed a torsion pendulum system for various noise tests with a resolution of \(10^{-11}\) m/s\({}^{2}\)/Hz\({}^{1/2}\). Huazhong University of Science and Technology in China used the high-voltage levitation technique and the torsion pendulum system to research the IS [28; 29]. The measurement resolution using the high-voltage levitation is \(4\times 10^{-8}\) m/s\({}^{2}\)/Hz\({}^{1/2}\), and the displacement resolution using the torsion pendulum can reach the order of magnitude of \(10^{-11}\) m/Hz\({}^{1/2}\). Furthermore, they performed the on-orbit functional verification in 2013 [30]. The Lanzhou Institute of Space Physics developed an engineering prototype and conducted high-voltage levitation experiments in an underground laboratory [31; 32]. The ground evaluated resolution of the IS can achieve \(2\times 10^{-7}\) m/s\({}^{2}\)/Hz\({}^{1/2}\). Based on years of MEMS experience, Tsinghua University has also carried out a lot of research work on electronic control systems and developed a high-voltage levitation based prototype with a resolution of \(4.21\times 10^{-7}\) m/s\({}^{2}\)/Hz\({}^{1/2}\)[33; 34; 35]. Because many technical requirements of the Taiji Project are difficult to assess with existing technology, it is necessary to carry out the key technology research and verification step by step.